This study was undertaken to determine the prognostic value of bone marrow histopathologic and molecular analyses in 53 patients with mycosis fungoides and 7 with Sézary syndrome. Bone marrow was involved in only 1 patient with Sézary syndrome, clinical stage IVA, before bone marrow biopsy. An ambiguous T-cell infiltrate was observed in 8 patients but was not associated with disease progression. The bone marrow specimen was normal in 51 patients. Monoclonality was detected in the skin specimen in 44 cases; an identical T-cell clone in the blood specimen was found in 21 of them and, in 16 of the 21 patients, in bone marrow specimens without histologic correlation. Multivariate analysis confirmed that clinical stage and detection by polymerase chain reaction of an identical T-cell clone in skin and blood specimens had an independent prognostic value. No further prognostic value was observed for the presence of a T-cell clone in bone marrow specimens. Our data do not support the need for bone marrow examination in patients with mycosis fungoides/Sézary syndrome.