The significance of partial regression in thin malignant melanomas (0.76 mm or less) of the skin was evaluated to determine if the regression was associated with the later development of metastases in patients who previously were considered to have a favorable prognosis. Of 575 patients with primary cutaneous melanoma treated and followed up by the Division of Surgical Oncology at the University of Illinois, Chicago, we found that 103 (18%) had tumors that measured less than 0.76 mm. Of these, 30 (29%) showed histologic evidence of partial regression. In six (20%) of the 30 patients, visceral metastases developed and the patients died. All six had more than 77% regression of their primary tumors. Of the remaining 24 patients, only one had regression greater than 77% and she is still alive three years after diagnosis. Most of these 24 (83%) patients had regression of less than 50% (mean, 29.9%). No metastasis occurred in the 73 patients who had thin melanomas without histologic evidence of regression. It is apparent from this study that patients with thin melanomas who show partial regression cannot be included in the "low-risk" group if the extent of regression is 75% to 80% or more.