Background The increased risk of second malignancies, including nonmelanoma skin cancers, in cutaneous T‐cell lymphoma (CTCL) patients has been well documented. However, relatively few studies of malignant melanoma in CTCL patients have been reported.

Methods A database of 250 CTCL patients registered over a 3‐year period was searched to identify patients with diagnoses of both mycosis fungoides (MF) and malignant melanoma.

Results We identified six cases of MF associated with malignant melanoma and one associated with dysplastic nevus syndrome, which is a marker of increased risk of melanoma. In four patients, melanoma was diagnosed along with or before MF. In the remaining two patients, MF was diagnosed prior to melanoma, although dysplastic nevi were noted at the time MF was diagnosed. These two patients received treatment for their MF (one with topical nitrogen mustard and another with radiation therapy and nitrogen mustard) prior to the histologic confirmation of melanoma. Six patients had early stages of MF (IA or IB), while one patient presented with simultaneous erythrodermic mycosis fungoides involving the lymph nodes as well as melanoma metastatic to the lymph nodes from an unknown primary.

Conclusion There is an elevated prevalence of malignant melanoma in MF patients compared to the general US population (P < 0.00001) with a relative risk of 15.3 for observing malignant melanoma in MF patients (95% confidence interval 7.0–33.8). Possible pathologic links between the two diagnoses include effects of mycosis fungoides therapies, immunosuppression secondary to mycosis fungoides, and genetic alterations in the p16 tumor suppressor protein.