Several reports have suggested that bilateral bone marrow biopsy is better than unilateral biopsy in staging non-Hodgkin's lymphomas. Therefore since 1975, bilateral iliac crest bone marrow biopsy has been part of our standard initial staging investigation of non-Hodgkin's lymphomas. The present study is to determine the relative value of bilateral marrow biopsy compared to the unilateral procedure. We studied 176 patients who had evaluable adequate bilateral biopsies and tumor histology. Among 57 patients with low-grade lymphoma, 7 (12%) were unilaterally positive for lymphoma and 25 (44%) were bilaterally positive. Among 119 patients with intermediate and high-grade lymphoma, 6 biopsies (5%) were unilaterally positive and 10 (8%) were bilaterally positive for lymphoma. Doing the second (contralateral) biopsy changed the disease stage as assessed by bone marrow biopsy in 6.1% of patients with favorable histologies and in 2.5% of patients with unfavorable histologies. These changes are small. Moreover, treatment would not have been significantly altered in the majority of patients as a result of the changes. We conclude that the routine use of bilateral trephine biopsy in initial staging of non-Hodgkin's lymphomas needs to be reappraised. We believe that its use should be limited to selected patients in whom the finding of bone marrow involvement may influence the choice of treatment.