In 17 patients with atopic dermatitis and in 17 healthy control persons, rosette-forming T cells from peripheral blood were investigated. The percentage of rosette-forming T cells in atopies was significantly lower (p < 0.0025) than in the control group. In patients with atopic dermatitis there was a significant correlation between IgE serum levels and the number of rosette-forming T cells. Increase in IgE level is correlated to a decrease in the number of rosette-forming T cells. It is suggested that the primary defect in the immunological system in atopies may be found in the thymus- or T-cell-mediated immunological system, and that this defect be characterized by a decrease or lack of T cells with suppressor function for IgE formation.