The authors reported a case of a child suffering from Cow's milk protein allergy, in which a soybean milk was administered for one month. The incorrect preparation of the formula determined a megaloblastic anaemia due to folate deficiency. The observation of very high levels of seric aminoacids during the acute phase of the disease was noteworthy. The therapy with folic acid and the introduction of protein hydrolysated formula milk determined a rapid normalization of the haematological picture. During the follow-up the administration of soybean protein caused anaphylaxis showing the contemporary presence of soybean allergy. The challenge test with cow's milk performed afterwards showed the restoration of the tolerance towards cow's milk protein.