In 28 patients the existence of deficiency in lactase activity was examined indirectly with lactose tolerance test (LTT). Patients were divided in three groups. The first group consisted of 12 patients with confirmed diagnosis of "Irritable-colon syndrome". The second was composed of 6 patients suffering from ulcerative colitis. The third examined group was control group made of healthy persons without any gastrointestinal or metabolic disorders. To exclude the possible of monosaccharide malabsorption, all patients underwent glucose-tolerance test (GTT). Milk tolerance questionnaire was performed in all examined patients. All clinical signs were observed for 12 hours after performing LTT. Following the statistical analysis it was found that there was greater frequency of lactose intolerance in two first groups (I and II), comparing with the results obtained in the control group. Low raise of glucose level (less than 1.11 mmol/l) in the majority of patients was connected with characteristic lactose-intolerance clinical signs and symptoms.