Bone marrow trephine biopsies were performed on 107 previously untreated patients with Hodgkin's disease (HD). Fifteen patients (14%) exhibited bone marrow involvement. These consisted of two of three patients (67%) with lymphocyte depletion, six of 27 patients (22%) with mixed cellularity, five of 64 patients (8%) with nodular sclerosis, and two who were unclassified. Twelve patients manifested a diffuse pattern of involvement; three, a focal pattern. In eight patients more than 70% of the marrow biopsy was replaced by Hodgkin's tissue, in one patient 50% of the marrow biopsy was replaced, and in six patients less than 30% of the marrow biopsy was replaced. Typical Reed-Sternberg (RS) cells were found in the trephine biopsies in 13 of the 15 patients and mononuclear RS variants in two. Bone marrow involvement was the only evidence of stage IV disease in 10 of the 15 patients. In addition to the 15 patients with initial involvement with HD, 11 patients without marrow involvement exhibited granulomas (six) and benign lymphocytic aggregates (five) in their trephine sections. Hematological parameters were studied in all pretreatment patients. Only in the nodular sclerosis group were these parameters useful in differentiating patients with and without Hodgkin's involvement of the marrow. Seventeen additional patients who had been previously treated at the time HD was demonstrated in their bone marrow were also studied. Large areas of necrosis were frequently seen in previously treated patients and one patient demonstrated cryptococcosis in the bone marrow.