Paraffin-embedded tissue of skin biopsy specimens taken retrospectively from 24 patients with cutaneous malignant lymphomas (CML) and 8 patients with cutaneous lymphoid infiltrates (CLI) and other dermatoses were studied retrospectively with PC10 immunostaining. The results show a statistical significant difference among PC10 indices for cutaneous genuine histiocytic lymphoma (CGHL), cutaneous germinal center cell-derived lymphomas (CGCCL), cutaneous peripheral T-cell lymphomas (CPTL), non-mycosis fungoides (MF) and Sezary's syndrome (SS), and MF when compared with those for CLI. There is a linear relationship between PC10 index and square root of PC10 density, both of which seem to have a parallel relationship to the severity of malignancy in CML. The nuclear volume of the positive tumor cell or lymphocyte with PC10 immunostaining may be also useful in differentiating CML from CLI.