The majority of published cases of prolymphocytic leukemia (PLL) have been of B-cell origin. Nineteen cases of PLL of T-cell type have been described, as has a single case of PLL having a surface phenotype with features of both B-cells and T-cells. This report presents a review of these cases and comparison with one case of T-cell PLL. By using specific monoclonal antibody technique, this case was subcategorized into helper-cell phenotype: E-rosette(+), SIC( -), Anti-T(+), Anti-B( -), Anti-monocyte(-), OKT3( +), OKT4(+), OKT6( -), OKTS(-), Ia(+), and Tdt(-). Cytochemical studies showed paranuclear acid phosphatase granules. Postmortem examination revealed a predominant T-cell zone infiltration by the leukemic cells in the spleen and lymph nodes, with involvement of multiple organs. The application of the monoclonal antibody technique, which can be standardized among different laboratories to subclassify lymphoproliferative disorders into functional subtypes, should lead to a better understanding and more effective treatment of this disease. Cancer 54:463-470, 1984. ROLYMPHOCYTIC LEUKEMIA (PLL) was described by P Galton et af.' as a variant of chronic lymphocytic leukemia (CLL). The clinical and hematologic features of PLL have been clearly defined.'.' Costello and associates have described the ultrastructure of the PLL cells.3 These cells are usually monoclonal B-cells with strong surface immunoglobulin. The first case of T-cell PLL was reported in 1973.4-5 Following this, an additional 18 cases of T-cell PLL have appeared in the l i t e r a t ~r e . ~. ~-' ~ In one case, the leukemic prolymphocytes possessed both B- and T-cell markers.16 The ultra~tructure,~.~ cytochem- istry6,7,9,10.1 1,13,14,16,17 and i m m u n ~l o g i c ~-~~~-' ~~' ~ fi ndings of T-cell PLL have been extensively described, but multiparametric immunologic studies using specific monoclonal antibodies in conjunction with postmortem examination have not been reported. This report presents the clinical, cytochemical, immunologic and ultrastructural findings of antemortem and postmortem tissue of a case of T-cell PLL with helper-cell phenotype in which From the Departments of Pathology and Internal Medicine, St. Louis