A prospective pretreatment staging evaluation was done on 49 consecutive patients with mycosis fungoides or the Sézary syndrome to study patterns of disease spread and prognostic factors. Routine staging procedures included complete blood count, blood chemistries, chest roentgenogram, lymphangiogram, radionuclide scans, bone marrow aspiration and biopsy, liver biopsy, and lymph node biopsy. Special evaluations included cytogenetic analysis, electron microscopy, and T-cell cytology. Extracutaneous lymphoma was documented by light microscopy in 51% of patients and by the three special procedures in 88%. Extracutaneous lymphoma was most frequent in blood and lymph nodes; 18% of patients had visceral involvement. Patients with generalized erythroderma had a higher frequency of extracutaneous disease than did patients with cutaneous plaques and tumors by both light microscopy and special studies. Survival was directly related to the type of skin involvement and the presence or absence of extracutaneous disease. Systemic dissemination of cutaneous T-cell lymphoma is frequent, generally asymptomatic, and develops early via the circulation. These findings may explain why cutaneous therapies are associated with a high frequency of relapse.