Progressively transformed germinal centers occurred in about 3.5% of cases of chronic nonspecific lymphadenitis. They are larger than germinal centers and are composed of follicular mantle lymphocytes, small clusters of proliferating mainly medium-sized B- and T-cells, as well as an extensive network of follicular dendritic cells. Sixty-six patients with lymph node enlargement containing progressively transformed germinal centers and staging and sequential biopsies of 213 patients with Hodgkin's disease (mixed and nodular sclerosis type) were investigated with special reference to the relationship of this lesion to Hodgkin's disease. In most cases, progressively transformed germinal centers developed without any obvious signs of illness and seemed to have no association with Hodgkin's disease. The patients could be differentiated into two groups. The larger group, Group 1 (n = 55 of 66) consisted of patients showing progressively transformed germinal centers without association to Hodgkin's disease. The smaller group, Group 2 (n = 11 of 66) showed progressively transformed germinal centers obviously with association to nodular paragranuloma (Hodgkin's disease lymphocytic predominance type). Progressively transformed germinal centers preceding (n = 3), simultaneously (n = 4), and after development of nodular paragranuloma (n = 4) were found. With regard to subtypes of Hodgkin's disease other than paragranuloma, progressively transformed germinal centers also could be found in sequential biopsies of Hodgkin's disease of mixed and nodular sclerosis type. In one case, progressively transformed germinal centers preceded, in another case they occurred simultaneously in mixed type of Hodgkin's disease, and in two cases of nodular sclerosis type progressively transformed germinal centers developed after the onset of Hodgkin's disease. These findings suggest that progressively transformed germinal centers may be a result of different processes that may be occasionally related not only to nodular paragranuloma, but also in rare cases to nodular sclerosis and mixed type of Hodgkin's disease.