Examinations of faeces (pH, Clinitest and Clinistix) are liable to contribute to the diagnosis of lactose malabsorption. To confirm the diagnosis the test is to be repeated while Lactase 500 is added to the child's milk. The enzyme does not split the lactase into the two monosaccharides, thus eliminating the malabsorption. The new drug Lactase 500 was used as a diagnostic parameter and for the treatment of lactose malabsorption in 20 infants. The effect on both groups was confirmed as mentioned by foreign authors. In addition to a very simple diagnostic method, which can be performed in the practitioner's office, there are two methods of treating lactose malabsorption: 1. feeding milk free of lactose, 2. feeding milk with an addition of Lactase 500. As in an earlier publication we assumed that about 10 per cent of the population suffers from secondary malabsorption of lactose, it seems advisable to check with the help of the described to simple methods all children with chronic diarrhoea.