Alopecia areata remains a disease of unknown cause and unsatisfactory treatment. Histologically it is characterized by a lymphocytic infiltrate that surrounds the lower half of the hair follicle. Our findings show that 64% to 92% of this infiltrate is composed of T lymphocytes bearing the Leu 3a (helper-inducer) phenotype. The remaining lymphocytes demonstrate the Leu 2a (cytotoxic-suppressor) phenotype. This helper T cell predominance is present in all patients, irrespective of clinical activity or duration of disease. The Leu 3a/Leu 2a ratio ranged from 2.6 to 19.8, the higher ratios being seen in patients with clinically active disease. The cutaneous infiltrate also is Ia-positive, indicating the activated nature of these lymphocytes.