BACKGROUND: Mycosis fungoides (MF) is the most common cutaneous T-cell lymphoma. Early-stage MF patches or plaques often resemble inflammatory skin disorders (ISDs), including psoriasis and atopic dermatitis. Cell adhesion molecule 1 gene (CADM1), which was initially identified as a tumor suppressor gene in human non-small cell lung cancer, has been reported as a diagnostic marker for adult T-cell leukemia/lymphoma. OBJECTIVE: We investigated CADM1 expression in MF neoplastic cells, especially during early stages, and evaluated its usefulness as a diagnostic marker for MF. METHODS: We conducted a retrospective study by using immunohistochemical staining and confirmed the expression of CADM1 in MF. In addition, we compared CADM1 messenger RNA expression in microdissected MF samples and ISD samples. RESULTS: In the overall study period, 55 of 58 MF samples (94.8 %) stained positive for CADM1. None of the 50 ISD samples showed positive reactivity (P < .0001). We found CADM1 messenger RNA expression in the intradermal lymphocytes of patients with MF but not in those of patients with an ISD. LIMITATIONS: We did not conduct a validation study for MF cases in other institutions. CONCLUSIONS: CADM1-positive cells can be identified in early stages with fewer infiltrating cells and may be useful as a diagnostic marker for early-stage MF.