Multicolor flow cytometry (FC) is indispensable for lymphoma diagnosis and classification, but its utility in evaluating skin biopsies for mycosis fungoides (MF) is not well established. We describe the largest series to date of skin biopsies evaluated by FC for MF (n = 33), and we compare the flow cytometric results with the histologic, molecular, and clinical findings. Abnormal T-cell populations were identified by FC in 14 of 18 patients (78%) having histologically confirmed MF and in no patient whose histology was negative or indeterminate for MF (n = 14). One patient had histologic, flow cytometric, and molecular findings compatible with MF, but this patient's clinical course was more suggestive of a drug eruption. Fourteen of 15 abnormal T-cell populations showed definitive aberrant expression of at least 2 surface antigens, including CD2 (47%), CD3 (67%), CD4, CD5 (87%), CD7, and CD45 (67%); most cases (67%) had light scatter properties suggesting increased cell size and/or cytoplasmic complexity. The high specificity of FC suggests that it will be a useful adjunct to routine histology in the evaluation of diagnostic skin biopsies for MF.