Background: Anemia is the most common hematologic problem worldwide especially in developing world. There are many causes of anemia in ICU patient such as gastrointestinal bleeding, anemia of chronic inflammation, trauma or surgery. Patient who were admitted in the ICU need to have a lot of blood tests related to their illness and for further treatment, however that can cause decrease of hematocrit or anemia. Objective: To identify the estimate volume of blood drawn from medical ICU patients and findout whether the volume of blood drawn effects the Hemoglobin/Hematocrit (Hb/Hct) or anemia and to identify the cause of anemia in ICU patient. Material and Method: This is an observation prospective cohort study collecting the volume of blood drawn from patients who were admitted to ICU Thammasat university hospital during 1st October 2007 through 31st March 2008. Results: This study found the mean volume of blood samples drawn from ICU patients is 77.8 ml. Sixty-eight percents of patients had a decrease Hb/Hct level during admission and around sixteen percent had anemia during admission. There is no statistically significant between the volume of blood drawn and the decrease of Hb/Hct. Conclusion: Decrease of Hb/Hct or anemia are common in ICU patient. This study showed that iatrogenic blood loss for laboratory investigation is one of the cause of anemia. We concluded that physician should order phlebotomy for necessary investigation for the most effective of diagnosis and treatment, but the less complication.