Cytostatic drugs are known to produce disturbances in intestinal absorption of carbohydrates. To further explore the gastrointestinal (GI) toxicity of cytostatic therapy, 37 patients with acute leukemia were investigated during and/or after remission induction courses by the use of the differential sugar absorption test (DSAT) and the intestinal clearance of alpha-1-antitrypsin (ClAAT). The ratio of the lactulose to the mannitol excretion in the urine was found abnormal in 44% of the tests. The ClAAT was increased in 74% of tests. The tests results differed considerably from patient to patient and depended on the chemotherapy course; correlation between the tests was low, probably indicating the unrelated pathophysiological processes were measured. After haematological regeneration, abnormal test results normalised. It is concluded that aggressive chemotherapy not only causes a reduction in the absorption of sugars, but commonly also protein leakage. These GI side-effects are reversible, and the application of both tests in combination provides a practical and reproducible method for investigation of GI toxicity in patients treated with cytostatic drugs.