Food allergy was investigated in 20 children with cystic fibrosis (CF), who still suffered from diarrhea and failed to thrive, in spite of adequate diet and enzyme treatment (group A). The study also included two age-matched control groups, comprising 10 CF children without intestinal symptoms and/or failure to thrive (group B), and 20 healthy children (group C). Skin tests were positive and total IgE higher than the mean + 2SD respectively in 14/20 and 11/20 patients of group A, in 3/10 and 2/10 patients in group B and in none in group C. The specific IgE were present in 6/14 children in group A whose skin tests were positive and in none in group B. There was no significant difference between group A and group B (p > 0.05). The levels of specific antibodies IgG, IgA and IgM were overall higher than the mean + 2SD of the normal in 18/20 in group A, in 6/10 in group B and in none in group C. The measurement by ELISA of specific antibodies for cow milk and egg proteins showed a statistically significant difference for casein, beta-lactoglobulin and ovalbumin between the IgG (p < 0.05) and IgA (p < 0.001) levels in group A and the other groups (B and C). Symptoms improved in 90% of CF patients (group A) when the implicated foods were eliminated from the diet and in 78% the oral provocation test resulted positive. The occurrence of food allergy must be considered in CF patients who do not improve with the conventional treatment. In these patients immunological investigations, in particular the measurement of IgG, IgA and IgM specific antibodies, are useful for diagnosis and in selecting an appropriate diet leading to an improvement in nutritional status.