To assess the frequency of occurrence and risk factors for multiple primary melanoma. Population-based, case-control study. New Hampshire. Three-hundred fifty-four New Hampshire residents with a confirmed first diagnosis of cutaneous melanoma. Diagnosis of a subsequent primary cutaneous melanoma. An additional melanoma occurred in 27 individuals (8%) within 2 years of their initial diagnosis, including 20 (6%) within the first postdiagnosis year. In 9 (33%) of these 27 cases, at least 1 subsequent melanoma was deeper than the first tumor. The 27 individuals with a subsequent melanoma diagnosis were classified as "cases" and were compared on the basis of risk factors to the 327 "controls" with a single melanoma diagnosis. The data indicate an inverse relation of risk of multiple primary melanomas with multiple blistering sunburns (P = .01 for the trend); the odds ratio (OR) was 0.32 (95% confidence interval [CI], 0.11-0.93) for 2 or more sunburns compared with none. The number of atypical moles was significantly related to increased risk (P = .004 for the trend). The presence of 3 or more atypical moles compared with none was associated with more than a 4-fold risk of multiple primary melanomas (OR, 4.29; 95% CI, 1.51-12.16). Additional melanomas occur more frequently than previously shown. Our study confirms that atypical moles are strongly associated with risk of multiple primary melanomas but provides little evidence that risk is influenced by pigmentary characteristics, hours of sun exposure, or benign moles. The inverse association with blistering sunburn may reflect the influence of an unmeasured covariate.