Cases of cutaneous T-cell lymphomas including Sézary syndrome were used to evaluate nuclear contour index (NCI), nuclear cytoplasmic ratio (N/C), and helper/suppressor T-cell ratio. It was found that: (1) the average NCI measured in pan-T-cell monoclonal antibody positive cells (N = 100) is higher than that measured in non-T-cells (N = 100) in the same lesion; (2) NCI increased from erythematous stage to tumor or Sézary stage as disease process advances; (3) N/C was not significantly increased with disease process and did not correlate with NCI; (4) suppressor T-cells were significantly increased in early stages of the disease, whereas helper T-cells were dominant in late stage lesions; and (5) the majority of tumor cell infiltrating into the epidermis were helper T-cells in all stages. In control specimens from chronic and acute skin conditions in which lymphocytic infiltration is seen, cells with NCI greater than 6.5 did not exceed 20%. It is concluded that if 25% or more cells in the lesion have NCI greater than 6.5 in pan-T-monoclonal antibody positive cells, the diagnosis of CTCL is very likely. Varying numbers of OKT6 positive cells (Langerhans' cell, thymocyte) were admixed in the dermis of all cases, and had no significant correlation with the stages.