The purpose of this study was to determine whether a new ultrafiltrated whey hydrolysate infant formula, Profylac®, could be administered safely to children with cow milk protein allergy/intolerance. Profylac has a stated molecular weight of < 8 kD and at least 30, 000 times reduced antigenicity which is controlled by a combination of ELISA-techniques and immunochemical methods. The study comprised 66 children with cow milk protein allergy/intolerance diagnosed by controlled elimination/ challenge procedures. The children were aged 1 month-14. 5 years, median 1% years and 15 were below 1 year. Thirty-five of these children had proven IgE-mediated reactions (cow milk protein allergy). Sixty-one of the children had at least two different symptoms and 31 had concomitant allergies to other foods and/or inhalants. All 66 children underwent and tolerated open, controlled challenges with Profylac. A total of 64 children continued having Profylac daily for at least 3 months and 58 for at least 6 months after challenge. Nine of the children older than 1 year did not like the taste and only had Profylac in minor amounts. No side effects were registered. Fifteen of the infants were below 1 year of age, and this group was compared with an age matched group of 16 infants challenged with and fed an extensively hydrolysed casein hydrolysate, Nutramigen®. All the infants in these two groups accepted and tolerated Profylac and Nutramigen, respectively, and no side effects were registered. Among the 35 patients with IgE-mediated reactions 6% (2/35) had positive skin prick tests and 11% (3/28) had specific IgE class 2 against Profylac, 2 of the latter before intake of Profylac. None of the patients with non-IgE-mediated reactions had a positive skin prick test or specific IgE against Profylac. The study provides 95% confidence that this product is tolerated by at least 95% of children with cow milk protein allergy/intolerance and by 90% with IgE-mediated reactions. We conclude, that this ultrafiltrated whey hydrolysate is generally safe to feed to children with verified adverse reactions to cow milk protein, including children with IgE-mediated reactions. The taste of the product was widely accepted, also by older children.