Mononuclear cells (MNC) present in the dermis of the forearm and in the blood of patients with progressive systemic sclerosis (PSS) were quantified and analyzed for subsets using monoclonal antibodies. The findings were correlated with cutaneous and systemic features of the disease. Total T lymphocytes and their subsets, B cells, and macrophages were enumerated in the skin samples of 21 patients with PSS. The dermal MNC infiltrates consisted mostly of activated T lymphocytes with a mean T helper/T suppressor (T4/T8) ratio of 2.4 +/- 1.3 SD. Few B1-positive or T6-positive cells (macrophages) were observed. There was no correlation between the skin or blood T4/T8 ratios and the degree of skin thickening. On histologic examination, 58 of 115 (50%) untreated patients with PSS had prominent dermal MNC infiltration. Significant correlations between the degree of MNC infiltration and both the degree (P less than 0.05) and progression (P less than 0.05) of skin thickening were observed. No correlations with other systemic disease features of PSS were noted. These results suggest that cutaneous T lymphocytes may play a role in mediating dermal sclerosis in PSS.