The incidence and degree of incomplete lactose absorption was investigated in breast fed infants and children up to two years of age during acute gastroenteritis (GE). Lactose absorption was assessed in 50 patients by means of the hydrogen breath test (HBT), approximately 5.5 days after the admission to hospital. HBT detected incomplete lactose absorption of marked (lactose malabsorption) and probably mild degree in 8 and 6 patients respectively. Incomplete lactose absorption appeared to be transient in all 5 patients retested after discharge. HBT failed to identify 8 cases of lactose intolerance which were detected by investigation of the stools. In 31 breast fed controls of a similar age range incomplete lactose absorption of only mild degree was probably present in 2 and lactose intolerance in 1, which too was only detected by investigation of stools. During acute GE the use of HBT is appropriate to detect milder forms of incomplete lactose absorption than lactose intolerance. For the detection of lactose intolerance the measurement of pH and reducing substances in the stools remains the method of choice. The findings are in favour of the continuation of breast feeding during acute GE.