Background: Having either mycosis fungoides or B-cell lymphoma may predispose a patient to the other.

Objective: We sought to determine whether the contemporaneous occurrence of the two malignancies is greater than chance and to investigate possible risk factors for the second malignancy.

Methods: We retrospectively reviewed the records of patients with contemporaneous mycosis fungoides and B-cell lymphoma seen between 1990 and 2007 at Mayo Clinic, Rochester, MN, or at Mayo Clinic, Scottsdale, AZ.

Results: In all, 23 patients had contemporaneous mycosis fungoides and B-cell malignancy. The first diagnosis was mycosis fungoides in 10 patients and B-cell lymphoma in 7; in 6 patients, the diseases were diagnosed simultaneously. No therapeutic factors could account for a predisposition to a second malignancy.

Conclusions: Retrospective design, referral center, and small sample size are limitations. Conclusions: Mycosis fungoides and B-cell lymphoma are unlikely to occur contemporaneously by chance, but no factor obviously predisposes a patient with one malignancy to development of the second.