Nasal secretion samples from 473 children aged 0--15 years were studied in order to evaluate the role of mast cells in the nasal smear in different types of atopy and food allergy. The occurrence of eosinophils in the nasal secretion and in the blood, and susceptibility to upper respiratory infection were recorded. A mast cell count of more than 20 cells per slide was indicative of atopy and there was a clear correlation between eosinophilia and mastocytosis in the nasal smear. No type of allergy seemed to be significantly predominant in the total material, but in young children aged 0--3 years mastocytosis was clearly related to food allergy. The examination of nasal smears for mast cells is a valuable test in diagnosing atopy in children and especially food allergy among young children. However, the test is not reliable enough to be used as a single screening test.