The occurrence of Hodgkin's disease (HD) and non-Hodgkin's lymphoma (NHL) appearing in the same individual indicates a closer relationship between the 2 diseases than previously believed. The purpose of our study was to analyze cases of HD and NHL in a defined population clinically, histopathologically and immunohistochemically to look for similarities indicating a common cellular origin. Between 1974 and 1994, 77 individuals were identified from the Swedish Cancer Registry and the National Health Care Programme for HD as potentially having both diagnoses. Thirty-two patients who had both HD and NHL were available for histo-pathological re-examination and immunohistochemical staining with CD30, CD15, LMP, p53, CD45 (LCA), CD3, CD45R0 (UCHL-1), L26, MB2 and CD45R (4KB5). The most common relation was HD preceding a high-grade malignant NHL (16 of 32 patients), unexpectedly often of T-cell phenotype (7 of 16 patients). The next common association was NHL of B-CLL type followed by HD (7 of 32 patients). At clinical presentation, the first lymphoma did not differ from lymphomas not associated with a second lymphoma, whereas the second one often appeared with a disseminated and aggressive clinical form. There was a significant correlation between the expression of p53 and LMP in first and second lymphomas. CD3 antibody was frequently expressed both in HD and NHL, whereas positivity for B-cell-related antibodies, CD30, CD15 and CD45R0, was less frequent and generally lower than previously described. The occurrence of HD and NHL in an individual is unusual. Tumour biological features common to both HD and NHL may indicate a similar cellular origin, regardless of the time interval between the diagnoses, and may contribute to the understanding of the pathogenesis of lymphoma.