The incidence of most epithelial cancers rises with a power of age. However, second breast cancers have a high constant incidence independent of age. The skin is one of the few other sites allowing examination of age incidence curves of second neoplasms of the same organ. We considered the risk of second primary cutaneous malignant melanoma (CMM) in a population-based series of 3,439 first CMM registered and followed-up between 1974 and 2003 in the Swiss Cantons of Vaud and Neuchatel (about 786,000 inhabitants). A total of 43 cases of second CMM were observed vs. 9.3 expected, corresponding to a standardized incidence ratio (SIR) of 4.6. The SIR was 8.5 under age 50, 5.7 at age 50-59 and 3.5 at age 60 or over. At 20 years, the cumulative risk of second CMM was 5%. Age-specific incidence rates of second primary CMM did not vary across age groups 30-39 through 80+, ranging between 1 and 2.5 per 1,000 person-years. Thus, the risk of CMM is substantially increased in subjects diagnosed with a CMM, and the relative risk is greater at younger age and declines with advancing age. The high constant incidence curve of second CMM is compatible with the occurrence of a single mutational event in a population of susceptible individuals. [Ed.]