The elective treatment for allergy to cow's milk protein is the elimination of these proteins from the diet. The present study with a follow-up of over two years took the form of a comparison between different replacement formulas based on soya (group A), hydrolysate of soya and bovine collagen (group B), and hydrolysate of casein (group C), randomly administered to 55 children (30 males and 25 females, aged between 2-48 months) with documented allergy to cow's milk proteins, but with different clinical symptoms. Tests to evaluate the acquisition of clinical tolerance to cow's milk proteins were performed using a day-hospital regime every 6 months. Sensitivity reactions were observed in 22% of cases in group A, 8% in group B and 37.5% in group C. It is worth underlining that 5 of the 6 children with reactions to soya protein then showed an excellent tolerance to hydrolysate of soya when it was administered subsequently until tolerance was achieved. Weight and statutory growth was uniformly good in all three groups. A high percentage of children achieved tolerance after 24 months (72%); the mean time taken to acquire clinical tolerance was 11.6 +/- 4.8 in group A, 11.6 +/- 6.02 in group B, and 14 +/- 5.6 in group C. No correlation was found between the type of initial symptoms, age at onset, method of response to first challenge and the time taken to acquire tolerance.