Steady-state bioavailability of sustained-release theophylline (SRT); Theo-Dur®, Uni-Dur® and Xanthium® were compared in 10 healthy males with theophylline clearance ranged from 0.3 - 0.8 ml/min/kg. Each of 400-mg SRT was administered once daily before breakfast for 7 consecutive days, one-week washout period in a crossover fashion. Serial blood samples were collected over 24 hours on days 6 and 7. Serum theophylline concentrations were determined by fluorescence polarized immunoassay. We found that the oral bioavailability relative to Franol®, (%F [90% CI]) of Theo-Dur®, Uni-Dur® and Xanthium® were 97 (93-106), 85 (79-96) and 77 (72-87), respectively. Average bioequivalence revealed that the Cssmin (μg/ml) of Uni-Dur® (5.07) was higher than Theo-Dur® (4.29), and Xanthium® (4.18), while the Cssmax and Cssav (μg/ml) of Theo-Dur® (11.02, 7.87) were statistically higher than Uni-Dur® (8.51, 6.91) and Xanthium® (7.65, 6.27). The extent of absorption assessed by AUCss0-24 of Theo-Dur® was significantly greater than Uni-Dur® and Xanthium®. However, fluctuation index (% FI) of Theo-Dur® (232) was twofold higher than Uni-Dur® (137) and Xanthium® (113). The median Tssmax of Uni-Dur® was 12 hours which was significantly longer than Xanthium® (7 hours) and Theo-Dur® (8 hours). There were no statistically significant differences between Uni-Dur® and Xanthium® regarding bioavailability, Cssmax, Cssav as well as % FI. Moreover, 400 mg OD of Uni-Dur® and Xanthium® are suitable for subjects with a theophylline clearance of 0.3-0.55 ml/min/kg while 400 mg OD Theo-Dur® can be used in subjects with slower clearance rates of 0.3-0.39 ml/min/kg. Subjects with rapid theophylline clearance rates of 0.65-0.8 ml/min/kg required a higher dose of theophylline and twice-daily dosing was more appropriate.