Systemic sclerosis (SSc) is a chronic inflammatory connective-tissue disease of unknown etiology, characterized by fibrosis and microvascular injury in affected organs. It has become clear that the activated cellular-immune system plays a central role in the pathogenesis of SSc. This study analyzes the numbers of lymphocyte subpopulations and their relations with clinical and laboratory manifestations. We studied a group of 42 patients with SSc and a group of 28 matched normal controls by flow cytometry using the lymphocyte cell-surface markers CD2, CD3, CD4, CD8, CD19, CD25, CD45RA, CD56, CD71, HLA-DR, TCR alpha/beta, and TCR gamma/delta. Patients with SSc had similar percentages of CD2+, CD3+, CD3+ CD4+, CD3+, CD8+, CD25+, CD4+ CD45RA+, CD8+ CD45RA+, CD71+ cells, and CD4+/CD8+ cell ratio when compared to normal controls. In contrast, the percentages of TCR gamma/delta cells were significantly lower in SSc patients with diffuse and late-stage disease with pulmonary involvement, muscle involvement, and the presence of anti-Scl-70 antibodies. Patients with diffuse SSc in early- and late-stage disease had significantly increased percentages of HLA-DR in CD4+ and CD8+ cells. Patients with late-stage disease had increased percentages of CD4+ CD45RA+ T-cells. Patients with limited and early-stage disease had smaller percentages of B-cells (CD19+). Patients with diffuse and late-stage disease had smaller percentages of NK-cells (CD56+). These results suggest that T-, B-, and NK-cell alterations may be involved in the onset of the disease, and/or in the perpetuation of disease, and may eventually be useful as a prognostic indicator in selected patient subgroups.