We investigated the populations of activated T (HLA-DR+CD3+) cells and natural killer (CD57–CD16+) cells in the peripheral blood of patients with various types of alopecia areata (AA) and noted any changes that occurred in the said populations after administration of local and systemic corticosteroid therapy. In type 2 (severe multiple AA and alopecia totalis) and type 3 (alopecia universalis), the mean percentages of HLA-DR+CD3+ cells and CD57–CD16+ cells were significantly higher when compared with those of the normal controls. The percentages of both subsets in type 1 (mild AA) and the normal controls were consistent. Twenty-four patients in types 2 and 3 had received corticosteroid treatment, and all patients experienced new hair growth. With the changes in disease activity, the populations of HLA-DR+CD3+ cells in these patients after corticosteroid therapy significantly decreased when compared with those recorded prior to treatment. Subsequent to treatment, the mean percentages of CD57–CD16+ cells decreased to levels that were not significant relative to that of the normal controls. These findings indicate that HLA-DR+CD3+ and CD57–CD16+ cells in the peripheral blood of patients with AA may be correlated with the disease activit