In order to define additional diagnostic criteria for the early diagnosis of Sézary's syndrome (SS), peripheral blood lymphocytes of 32 patients with erythroderma, including 8 patients with SS, 4 patients with erythrodermic mycosis fungoides, 14 patients with an erythroderma on the basis of atopic or chronic dermatitis, and 6 patients with erythrodermic psoriasis, were investigated by computer-assisted morphometry. The degree of nuclear indentation, expressed as the nuclear contour index (NCI), was measured on electron micrographs. The mean NCI and the percentages of cerebriform mononuclear cells (CMC), defined by a NCI greater than or equal to 6.5, were calculated. In addition, the percentages of lymphocytes, T and B cells, and the distribution of T-cell sub-populations as defined by Fc-receptors (T mu, T gamma) and monoclonal antibodies (OKT3, OKT4, OKT8, HLA-DR) were determined. Statistical analysis showed as most discriminating parameters for the differentiation between SS and benign forms of erythroderma: high percentages of lymphocytes (50% or more), an expanded OKT3+, OKT4+ population with an OKT4/OKT8 ratio greater than 10, a mean NCI value greater than or equal to 5.5, the presence of more than 20% CMC, as well as the presence of cells with a NCI greater than or equal to 11.5. The total leukocyte and lymphocyte counts, as well as the percentages of B, T, T mu, and T gamma cells had limited value for the early diagnosis of SS.