It has been suggested that patients with thin malignant melanoma displaying evidence of histological regression may have a poor prognosis. In the present study, the case histories of 353 patients with clinical stage I cutaneous malignant melanoma up to 0.7 mm thick were reviewed to determine if either active or past regression in these lesions was a poor prognostic sign. Lesions were reported as displaying evidence of partial regression if either (a) a portion of the melanoma had a heavy lymphocytic infiltrate associated with loss of tumour cells or the presence of degenerating tumour cells, or (b) a portion of the melanoma was replaced by vascular fibrous tissue with or without pigment-containing phagocytes. The incidence of regression in this study (58%) was similar to that reported in another recent large study on thin lesions (53%). Only slightly more regressed than unregressed lesions metastasized (8% versus 5% respectively). A high proportion of first recurrences from these thin lesions developed at sites remote from the primary lesion (lung, bone or in subcutaneous tissues or lymph nodes wide of the line of spread). However, the presence or absence of regression in thin lesions did not appear to influence the site of first recurrence. Cumulative 10-year survival rates for patients whose lesions displayed or did not display evidence of either active or past regression were nearly identical.(ABSTRACT TRUNCATED AT 250 WORDS)