Although the majority of circulating leukemic cells in chronic lymphocytic leukemia (CLL) are in G0/early G1, recent studies have shown that these cells have undergone multiple cell divisions. In this study, we have determined whether there are abnormalities in cell cycle control in CLL by examining the three cyclin D isoforms in 43 patients and correlating the findings with clinical features. Cyclin D mRNA was measured by a sensitive RNase protection assay and the order of expression in CLL cells was D3 > D2 > D1. The mean cyclin D1 and D3 mRNA levels were 4 to 6-fold higher in CLL cells than in normal peripheral blood B cells. In contrast, the levels of cyclin D2 mRNA were similar in CLL and normal B cells. Expression of the cyclin D isoforms was two- to four-fold greater in normal T cells than B cells, and the order of expression for both cell types was D2 > D3 > D1. The relative overexpressions of cyclins D1 and D3 in CLL were unrelated to gene amplification, as assessed by Southern blotting, but structural changes in the genes were seen in four patients. Both cyclin D1 and D3 mRNA levels correlated positively with lymphocyte doubling time (LDT) and inversely with Rai stage and duration of disease. In addition, a significant correlation was observed between cyclin D mRNA levels and survival, with patients having high levels of cyclin D1, and to a lesser extent cyclin D3, mRNA having the best survival. Thus, cyclin D1 and D3 are relatively overexpressed in CLL cells and patients with higher levels have low stage disease, long LDT and prolonged survival. Further studies should evaluate the predictive value of cyclin D measurements in comparison to other prognostic markers in CLL.