Purpose: New primary melanomas (NPMs) have developed in some patients with metastatic melanoma treated with BRAF inhibitors. We sought to determine the background incidence of spontaneous NPMs after a diagnosis of American Joint Committee on Cancer/International Union Against Cancer stage III or IV melanoma in patients not treated with a BRAF inhibitor. Patients and Methods: Patients diagnosed with stage III or IV melanoma at Melanoma Institute Australia between 1983 and 2008 were analyzed, and those who received a BRAF inhibitor were excluded. Results: Two hundred twenty-nine (5%) of 4,215 patients with stage III melanoma and 43 (1%) of 3,563 patients with stage IV melanoma had at least one NPM after diagnosis of stage III or IV disease. The 6-month, 1-year, and 10-year cumulative incidence rates of developing an NPM after stage III melanoma were 1.2% (95% CI, 0.86% to 1.51%), 1.8% (95% CI, 1.44% to 2.26%), and 5.9% (95% CI, 5.08% to 6.74%), respectively. The 3-month, 6-month, and 1-year cumulative incidence rates of NPM after diagnosis of stage IV melanoma were 0.2% (95% CI, 0.07% to 0.36%), 0.3% (95% CI, 0.15% to 0.51%), and 0.4% (95% CI, 0.25% to 0.7%), respectively. In both patients with stage III and stage IV melanoma, male patients and patients with a prior history of multiple primaries had a higher incidence of NPM. Conclusion: Patients with stage III and stage IV melanoma remain at risk for development of further primary melanomas, particularly if they have a history of multiple primary melanomas before stage III or IV disease. The incidence rates are lower than those reported in patients receiving BRAF inhibitors. However, the results must be compared with caution because dermatologic assessment is more frequent in BRAF inhibitor trials.