Nine patients had composite lymphoma in which Hodgkin's disease (HD) and non-Hodgkin's lymphoma (NHL) involved the same anatomic site. Two of these patients had relapses of their tumors. In one, the initial biopsy specimen contained follicular and diffuse large cell NHL with unclassifiable HD, but the relapse showed diffuse large cell NHL with nodular sclerosis HD. In the other patient, both biopsy specimens showed follicular mixed NHL; the HD component in the initial biopsy specimen was nodular sclerosis, whereas, at relapse, it had the appearance of interfollicular HD. In the remaining seven patients, the HD component was subclassified as nodular sclerosis (three specimens) or mixed cellularity (three specimens), or it was unclassifiable (one specimen). The NHL component was categorized as diffuse large cell (two specimens), diffuse large cell immunoblastic (two specimens), follicular and diffuse large cell (one specimen), diffuse mixed small and large cell (one specimen), and lymphocytic lymphoma of intermediate differentiation (modified Rappaport classification) (one specimen). Paraffin section immunoperoxidase studies were done on the NHL component in eight patients (nine specimens) and on the HD component in six patients (seven specimens). In each of these, the NHL component was leukocyte common antigen (LCA) positive and Leu-M1 negative. In addition, the neoplastic cells were L26 positive and UCHL-1 negative, indicating a B-cell phenotype. In five of seven immunophenotyped cases, Reed-Sternberg (RS) and Hodgkin's (H) cells from the HD areas were Leu-M1 positive and LCA negative, reflecting an immunophenotype that is typical of non-lymphocyte-predominant HD. In two specimens, the malignant cells were negative for Leu-M1 and LCA (with positive internal controls). Composite lymphomas composed of HD and NHL are unusual, and cases of coexistent HD of the non-lymphocyte-predominant subtype and NHL are even less common. The results of the current study and a review of the literature indicate that this phenomenon usually involves a B-cell NHL that coexists with HD, perhaps further suggesting a close relationship between the malignant cells of HD (RS and H cells) and B lymphocytes.