Approximately 30% of multiple myelomas (MMs) express cyclin D1 when assessed using immunohistochemical techniques. Cyclin D1 expression correlates with greater tumor burden in MM, because cyclin D1-positive cases are more frequently associated with extensive bone marrow involvement, i.e., high pathologic stage, than are cyclin D1-negative cases. The mechanisms that explain this association are unknown. To explore other differences between cyclin D1-positive and cyclin D1-negative MMs, we assessed 59 MMs immunohistochemically for several G1 cell-cycle regulatory proteins, including cyclin D1, E2F-1, p53, mdm-2, and p21waf-1, using routinely fixed and processed, paraffin-embedded bone marrow specimens. Twenty MMs (34%) were cyclin D1 positive, and 39 (66%) were cyclin D1 negative. Eighteen (90%) of 20 cyclin D1-positive MMs were Stage III, in contrast to 19 (49%) of 39 cyclin D1-negative MMs (P = .003). Cyclin D1-positive MMs were more likely to express E2F-1 (16/20 vs. 4/39, P < .001), p53 (11/20 vs. 10/39, P = .041), and p21waf-1 (12/20 vs. 7/39, P = .003). There was no significant difference in mdm-2 expression between these groups. We also assessed proliferation rate using an antibody specific for the Ki-67 antigen. A relatively high percentage (> 20%) of Ki-67-positive cells was found in cyclin D1-positive MMs compared with cyclin D1-negative MMs (13/20 vs. 3/39, P < 0.001). These results suggest that cyclin D1-positive MMs are more likely to possess additional derangements involving other G1 cell-cycle regulatory proteins. We speculate that these abnormalities might result in increased proliferation, thereby explaining the correlation between cyclin D1 expression and greater tumor burden.