486 patients with primary cutaneous malignant melanoma clinical stage I were examined in order to evaluate the prognostic importance of partial regression in thin lesions. All the melanomas measured 1 mm or less in maximal thickness. The study showed that past regression with fibrotic scar tissue adversely affected survival in patients with thin melanomas. The 10 year survival was 95% for patients without regression in contrast to 79% for patients with past regression. It was, furthermore, demonstrated that active regression without fibrotic scar tissue did not influence survival significantly. The wider and the thicker the fibrotic area, the poorer the survival. Although the prognostic importance of this finding was not statistically significant, we suggest that the horizontal width of the fibrotic area in particular may be a valuable prognostic guide in thin melanomas with past regression.