Chromosomes were studied in cells from tissues primarily involved by diffuse "histiocytic" lymphoma in nine patients. Two of the patients had stage II disease; their tumors were fibrotic and had no mitotic cells. One patient was in stage III, and the remaining six patients had stage IV disease. The modal chromosome number of abnormal cells from these last seven patients was hypodiploid in two, hyperdiploid in four, and near-triploid in one. Complete banding studies of six cases and partial analysis of the seventh indicate that (1) every patient had a distinct cell line with common markers, with a few cells showing minor variants; (2) although certain chromosomes (Nos. 1, 2, 3, 9, 12, and 14) were structurally affected more often than others, no markers with the same banding pattern were noted among them; and (3) the cytologic type of lymphoma could be correlated with the karyotype in all seven patients. When the Lukes and Collins classification was used, three patients whose tumors were composed predominantly of large noncleaved cells showed a 14q translocation leading to the formation of a 14q+ marker chromosome. This marker was not observed in four patients whose tumors had a majority of large cleaved cells. These preliminary results, if confirmed in a larger series of patients, will provide additional evidence that there are consistent chromosome changes associated with specific subtypes of lymphoproliferative disorders analogous to the Ph1 chromosome in chronic myelogenous leukemia.