Paired biopsy samples from involved and uninvolved skin were obtained from 19 patients with generalized scleroderma (11 with early, progressive disease and 8 with late, improving disease). Skin biopsy samples were double stained for mast cell granules and for mast cell membrane. The number of mast cells was increased in patients with systemic sclerosis (SSc), in both involved and uninvolved skin and in both early and late disease. There was an increase in the number of degranulated mast cells in the involved skin of patients with both early and late disease and in the not-yet-involved skin of patients with early disease; however, there was no increase in the number of degranulated mast cells in areas of previously involved but now normal skin of patients with late disease. Increases in mast cell number and degranulation precede clinically apparent dermal fibrosis in SSc. These observations and the absence of mast cell degranulation in regressing skin suggest a participatory role of the mast cell in the clinical progression of skin changes in SSc.