Pretreatment bone marrow specimens from 273 patients with multiple myeloma (MM) and 148 patients with nonmyelomatous plasmacytosis were studied retrospectively to evaluate the usefulness of bone marrow sections and smears in establishing the diagnosis of MM. The combined findings in the sections and smears confirmed the clinical impression of MM in 83% of myeloma patients: in 59%, both sections and smears were diagnostic; in 17% the smears were diagnostic, the sections nondiagnostic; in 7% the sections, but not the smears, were diagnostic. In the remaining 17% of myeloma patients, both the sections and the smears failed to confirm the clinical impression of MM. Of the 148 patients with nonmyelomatous plasmacytosis, a pathologic over-diagnosis of MM was made in 2 patients. Reasons for these discrepancies, and problems relating to the validity of the current diagnostic criteria for MM are discussed. It is concluded that both bone marrow sections and smears should be examined when attempting the diagnose MM since either, by themselves, may not be diagnostic. Recommendations are made that immunoperoxidase staining methods be used in difficult cases, and that the marrow findings be correlated with clinical, laboratory, and radiologic findings prior to establishing the diagnosis of MM.