In a double blind study, 40 infants with cows' milk intolerance of various causes were randomised to receive a nutritionally complete formula in which nitrogen was supplied either as whey hydrolysate or amino acids. The median age of infants was 10 weeks (range 36 weeks' gestation to 108 weeks' postnatal age). After a median follow up period of 25 weeks there was no significant difference in dietary intake between the formulas. Twenty four weeks after entry, weight and weight for length improved equally on both formulas. Plasma albumin improved significantly on the hydrolysed whey formula but not in the amino acid group. Both milks were palatable and normal intakes of formula were maintained. Biochemical and haematological indices remained within normal limits. There was no difference in stool frequency and vomiting between the two formulas. Two infants developed a probable allergic colitis while receiving hydrolysed whey. Amino acid formula may have a role in the management of atopic infants with severe cows' milk intolerance who have already reacted to whey or casein hydrolysate formula.