We have evaluated the usefulness of bilateral rather than unilateral posterior iliac spine trephine biopsies in searching for lymphoma and other neoplastic diseases in the bone marrow. Two hundred and eighty-two patients with these diseases were studied. Tumor was found on only one side in 22% of patients with non-Hodgkin's malignant lymphoma, in 43% of patients with Hodgkin's disease, and in 36% of patients with other neoplastic processes. Thus, the second biopsy yields an additional 11% to 22% of positive biopsies. We conclude that bilateral trephine bone marrow biopsies should be routinely performed when searching for tumor in the bone marrow.