Caribbean T-cell lymphoma/leukemia (CATL) is a recently described neoplasm similar to Japanese adult T-cell leukemia/lymphoma, but it occurs in patients born in a different geographic region. Four adult Black West Indian women with CATL were studied using a multiparameter approach, including clinical, hematologic, pathologic, and immunologic investigations. The disease was usually characterized by adenopathy (with or without hepatosplenomegaly), peripheral blood involvement, hypercalcemia often in the absence of gross bony lesions, and a poor prognosis. The neoplastic cells were pleomorphic with marked nuclear irregularity of a multilobated or more cerebriform type. Lymph node biopsies demonstrated a diffuse pleomorphic lymphoma with variable degrees of nuclear irregularity and transformation. Marrow involvement consisted of focal nodules or, more commonly, scattered atypical lymphoid cells. Marrow biopsies also often demonstrated osteoclastic activity. Cases were all of T-cell origin, most commonly of T-helper phenotype, although there was some phenotypic heterogeneity. All patients had antibodies to the human T-lymphoma/leukemia virus.