The diets of 20 children with cystic fibrosis were analysed for energy and nutrient content with simultaneous measurement of energy losses in stools. Median energy intakes were in excess of the WHO estimated daily requirements (118.2%) when expressed as MJ/kg/24 hours, the excess almost accounted for by energy losses in the stools. When expressed as MJ/24 hours, however, median energy intakes were 98.7% of that estimated for normal children of median weight for age. Compared with recently published data for normal school children the fat content of the diet was reduced (30.0%) as were intakes of iron and zinc. Children whose whole milk intakes were high had the greatest amount of fat and energy in their diets and were able to absorb energy in excess of that recommended. We conclude that many children with cystic fibrosis are still on low fat diets and whole milk is the single most useful food for the provision of extra dietary fat and energy.