In 100 infants and children with bacterial or viral infections determination of faecal fat was done. None of the patients showed signs of malabsorption prior to investigation. The faecal fat content is expressed as percent of the dry weight of the stool. A fat content of up to 14% of the dry weight of the stool can be regared as normal. A faecal fat content of 15 to 20% is indicative for an elevated fat excretion, and a fat content of more than 20% gives evidence for steatorrhoea. In 80% of the infants with bacterial and in 50% of the infants with viral infections steatorrhoea could be demonstrated. In children with bacterial infections 18% showed a normal, 69% an elevated fat excretion ain 13% there was a steatorrhoea. In children with viral infections 6% showed a normal fat excretion whereas 81% had an elevated excretion and 13% exhibited steatorrhoea. Thus viral infections compared with bacterial infections seem to cause disturbances of fat absorption more frequent. Whereas the degree of the disturbance seems ot be less intense in viral infections.