Ten cases of urinary tract infections were treated with amoxycillin in a dosage of 3 times 375 mg (orally) for 10 days. The treatment was successful in nine cases. The laboratory control was done before commencement of the treatment, at the end and one month later. The serum concentrations in all patients as well as the quantity excreted in the urine were determined 1, 2, 4 and 6 h after the administration of the antibiotic. None of these cases presented any side-effects.