Five patients with atopic dermatitis (AD) who were sensitive to hen's egg were observed before and after natural measles virus infection. Within 4 weeks of natural measles virus infection, the eczematous lesions clearly improved in four of the five patients in whom neither offending foods were eliminated, nor anti-allergic drugs, systemic steroids and steroid ointment administered. This was accompanied by reduced proliferative responses of peripheral blood mononuclear cells (PBMCs) to ovalbumin (OA). Another patient showed a transient improvement of AD symptoms, from severe to mild, and thereafter returned to severe accompanied by increased proliferative responses of PBMCs to OA. Radioallergosorbent test (RAST) scores for hen's egg in all five patients did not change in each level in each patient, except the transiently decreased RAST scores for hen's egg in one patient, after the infection. Thus, in patients with AD who are sensitive to food, the improvement of AD symptoms that appeared within 4 weeks of natural measles virus infection was related to reduced proliferative responses of PBMCs to the food antigen following the infection.