The objective of this study was to determine the incidence of multiple primary melanomas (MPM) and other cancers types among patients with melanoma. Factors associated with development of MPM were assessed in a post hoc analysis of the database from a multi-institutional prospective randomized trial of patients with melanoma aged 18 to 70 years with Breslow thickness 1 mm or greater. Disease-free survival (DFS) and overall survival (OS) were evaluated by Kaplan-Meier analysis. Forty-eight (1.9%) of 2506 patients with melanoma developed additional primary melanomas. Median follow-up was 66 months. Except in one patient, the subsequent melanomas were thinner (median, 0.32 mm vs 1.50 mm; P < 0.0001). Compared with patients without MPM, patients with MPM were more likely to be older (median age, 54.5 vs 51.0 years; P = 0.048), to have superficially spreading melanomas (SSM) (P = 0.025), to have negative sentinel lymph nodes (P = 0.021), or to lack lymphovascular invasion (LVI) (P = 0.008) with the initial tumor. On multivariate analysis, age (P = 0.028), LVI (P = 0.010), and SSM subtype of the original melanoma (P = 0.024) were associated with MPM. Patients with MPM and patients with single primary melanoma had similar DFS (5-year DFS 88.7 vs 81.3%, P = 0.380), but patients with MPM had better OS (5-year OS 95.3 vs 80.0%, P = 0.005). Nonmelanoma malignancies occurred in 152 patients (6.1%). Ongoing surveillance of patients with melanoma is important given that a significant number will develop additional melanoma and nonmelanoma tumors. With close follow-up, second primary melanomas are usually detected at an early stage.