Twenty young adult atopic patients and their matched controls were studied. Spontaneously generated and Con-A-induced suppressor T cell functions as well as Natural Killer (NK) activity against K-562 target cells, measured in a short-time 3H-thymidine uptake, were evaluated. Suppressor T cell activity in the patients was more than 2 SD lower than that found in the controls and there was, contrary to expectation, a direct correlation between suppressor function and serum IgE levels. Atopic patients showed a statistically significant lower NK activity than normal controls when related to a low IL-2 production. Both facts inversely correlated with the concentration of IgE in serum. We concluded that atopic patients' vulnerability to viral infections may be due to defective NK activity. Suppressor T cell function is abnormal in these patients. Both defects could be due to a faulty immunoregulatory helper function.