In this study, we analyzed 69 plasma cell neoplasms, including 54 multiple myelomas (MMs), 3 cases of plasma cell leukemia, and 12 plasmacytomas, for expression of cyclin D1 protein using an immuno-histochemical method applied to routinely fixed, paraffin-embedded tissue sections. Cyclin D1 was expressed in 18 (26%) of 69 plasma cell neoplasms, including 16 (30%) of 54 MMs and 2 (17%) of 12 plasmacytomas. Three cases of plasma cell leukemia were negative. Cyclin D1 expression correlated with the cytologic differentiation and histologic stage of MM. Twelve (75%) of 16 MMs had intermediate or immature cytologic features and were histologic Stage III. With use of a polymerase chain reaction assay, we also analyzed six cases (three cyclin D1 positive, three cyclin D1 negative) for the t(11;14); one MM carried the t(11;14) and expressed cyclin D1 protein. We conclude that cyclin D1 expression occurs in approximately one-quarter of plasma cell neoplasms and correlates with the degree of cytologic differentiation and histologic stage. The relatively high frequency of cyclin D1 expression, compared with the less than 5% incidence of the t(11;14) detected by conventional cytogenetics reported in the literature, suggests that upregulation of cyclin D1 protein might be the result of mechanisms other than the t(11;14).