Ulceration of a cutaneous melanoma on microscopic sections is an adverse prognostic finding. The five-year survival rate is reduced from 80% for non-ulcerated melanomas to 55% in the presence of ulceration for Stage I melanoma patients and from 53 to 12% for Stage II melanoma patients (P less than 0.001). As a group, ulcerated lesions are thicker and more likely to have a nodular growth pattern. However, survival rates were still worse for ulcerated melanomas when matched with nonulcerated lesions for thickness and stage of disease. The width but not the depth of surface ulceration significantly correlated with survival. The median ulcer depth was 0.08 mm (range 0.01-1.2 mm). In those few lesions with ulcer craters more than 0.2 mm in depth, the melanomas were so thick they had the same poor prognosis regardless of whether thickness was measured to the base of the ulcer or to the top of the lesion. The Breslow microstaging method of measuring thickness is therefore a valid prognostic indicator, even for ulcerated lesions. The incidence of ulceration for the entire patient group ranged from 12.5% for melanomas less than 0.76 mm thickness to 72.5% for melanomas greater than 4.0 mm thick (P of correlation = 0.0001); from 12% for Level II invasion to 63% for Level V lesions (P = 0.005); from 23% for superficial spreading growth patterns to 49% for nodular and 74% for polypoid lesions (P = 0.0001); and from 27% for lesions with a heavy lymphocyte infiltration to 60% for minimal or absent host response (P = 0.005). There was no significant correlation with anatomic location, pigmentation of the melanomas, or with the patient's age and sex. Since ulceration appears to have such an important influence on survival rates, this parameter should be considered as a stratification criterion in clinical trials and accounted for when analyzing results of melanoma treatment.