Malabsorption of carbohydrates is a common consequence of illness among infants and young children. Verification of this clinical entity ensures amelioration of its attendant diarrhea by avoiding the offending carbohydrates. Standard tests for malabsorption of carbohydrate in clinical practice include determinations of reducing substances and /or pH in stool and /or breath H(,2). They are based on the fact that malabsorbed carbohydrates are reducing substances which undergo bacterial fermentation in the colon to produce organic acids and H(,2). The purpose of this study was designed (1) to develope a better test for stool reducing substances than the standard Clinitest method, (2) to test the usefulness of quantitation of stool reducing substances, stool pH and breath hydrogen as index of carbohydrate malabsorption and (3) to evaluate lactose, maltose, sucrose and dextrin with respect to tolerability in infants with acute and chronic diarrhea. The present study has followed the currently accepted criteria to indicate malabsorption of carbohydrates. They are reducing substances in stool > 0.5 g/dl, stool pH <5.5 and breath H(,2) > 20 ppm above baseline value. The method involved in the determinations of stool reducing substances, stool pH and breath H(,2) are as follow. Stool reducing substances: A new method for the determination of stool reducing substances has been developed. It is the "4-tube Benedict" test which is a modification of the 4-tube Benedict test used for semi-quantitation of sugar in spinal fluid. The procedure encompasses adding 0.1,0.2, 0.3 and 0.4 ml of 1:75 diluted stool to each respective tube containing 1.0 ml of Benedict solution. The tubes are then boiled for 5 minutes followed by 5 or 10 minutes of centrifugation at 2000 rpm. Appearance of cuprous oxide precipitate in 0,1,2,3 and 4 tubes is read as -,+,++,+++ and ++++ corresponding to less than 500,500,750,1000 and 2000 mg of reducing substances per 100 ml stool respectively in similar fashion as the results obtained with the Clinitest method. The 4-tube Benedict test and Clinitest gave clinically similar results in 96.6%.Storage of stools up to 8 hours at room temperature did not affect the results of the test. Predigestion of stools in 2N H(,2)S0(,4) for 30 minutes was essential to ensure correct diagnosis of mild to moderate carbohydrate malabsorption. Stool pH : Locally available pH paper were found to be good enough for discriminating pHs < 5.5 from pHs over 5.5 and thus could be used in clinical practice to detect carbohydrate malabsorption. Dilution of stool to 1:2 did not significantly change stool pH. Stool pH decreased with increasing time of storage both at room temperature and 4 C. It is, therefore, necessary to determine stool pH on fresh specimen. Breath H(,2): The present study has succeeded in developing a very simple yet reliable method of collecting expired air. It employed a 60 ml plastic syringe with a hole punched above the 40 ml mask connected to an oxygen face mask which was so placed to cover the nostrils and mouth. The procedure agitated the infant to scream. Air in the dead space of the respiratory tract was expelled out into open air during the beginning of screaming through the hole of the plastic syringe and was displaced in the syringe by expired breath during the end of screaming. After 30 seconds of screaming the syringe was disconnected from the face mask and the plunger of the syringe was pushed down below the hole to the 40 ml mark. In this way expired breath was collected and analyzed for hydrogen concentration with gas chromatographic instrument. The precision of this method was tested by comparing the present method of breath collection with that employing a "Quintron" storage bag in 20 infants and in 5 normal healthy adults following ingestion of lactose and for loss of hydrogen following storage of expired air up to 8 hours at room temperature in 24 infants. The results showed that concentration of breath hydrogen remained constant with collection time of 30 seconds,did not differ significantly whether expired air was collected with a plastic syringe or a "Quintron" storage bag and did not change statistically following storage time prior to determination of up to 8 hours at room temperature. Thus it is concluded that the method of gas collection developed in the present study is reliable yet very simple and determination of breath H(,2) is practical and easy. Test of choice for carbohydrate malabsorption: The usefulness of three tests for carbohydrate malabsorption was evaluated in 30 infants with acute diarrhea and 5 infants with chronic diarrhea following ingestion of four carbohydrates at 2 g per kg body weight. Each of the former was randomly fed two carbohydrates, one on each day on two consecutive days, while each of the latter was fed all four carbohydrates also in random fashion one on each day on four consecutive days. Breath H(,2) was determined at 0,l,2,3,3 1/2, 4, 4 1/2, 5 and 6 hours following ingestion of carbohydrate. Any stool passed during this 6 hour period was determined for pH with pH papers and reducing substances in undigested and digested samples with the 4-tube Benedict test. The results showed that test for stool reducing substances was the most sensitive method for diagnosis of carbohydrate malabsorption in clinical practice. Determination of stool pH and breath H(,2) contributes little to the diagnosis of carbohydrate malabsorption. Furthermore, lactose was found to be the least tolerated carbohydrate among infants with diarrhea when compared to dextrin, maltose or sucrose.