The dermatopathologic findings in cases of Sézary syndrome (SS) that arise in patients without a previous diagnosis of mycosis fungoides have not been well characterized. We evaluated the histologic findings in skin biopsy specimens from 31 patients with such primary SS and correlated them with clinical and hematologic parameters at the time of biopsy. The most characteristic histologic finding was the presence of a dermal perivascular lymphoid infiltrate, usually with mild to moderate cytologic atypia and variable numbers of eosinophils; epidermotropism was absent or minimal in 19 cases (61%). Reactive epidermal changes such as spongiosis, parakeratosis, and acanthosis also were present frequently (27 [87%], 17 [55%], 19 [61%] cases, respectively). The number of eosinophils present in skin biopsy specimens correlated with the level of peripheral blood lymphocytosis. In erythrodermic patients or patients with persistent xerosis and pruritus, it is important to carefully evaluate the degree of lymphocyte atypia in the dermal perivascular infiltrate and correlate with blood flow cytometric findings to diagnose primary SS. Many cases will lack the epidermotropism usually seen in mycosis fungoides.