Lymphocyte subpopulations were determined in the blood of patients with atopic dermatitis (AD) before and after treatment with the thymopoietin pentapeptide TP-5. The relative and absolute numbers of lymphocytes bearing suppressor/cytotoxic cell markers (FcIgG⁺E⁺ and T8⁺ cells) were significantly decreased in the untreated patients and the T4⁺/T8⁺ cell ratio was increased, indicating an imbalance between lymphocyte subpopulations in AD. Patients who had been treated for 6 weeks with TP-5 displayed no significant abnormality of any of the lymphocyte subsets studied and comparison of pre- and posttreatment values revealed that there was a statistically significant increase in T8⁺ cell numbers, that by contrast did not take place in placebo-treated AD patients. The treatment had no demonstrable effect on IgE serum levels or on the spontaneous in vitro production of IgE by cultured lymphocytes from the patients.