The prognostic value of lymphocyte doubling time (LDT), expressed in months and obtained by means of a linear regression, has been studied in 99 previously untreated chronic lymphocytic leukemia (CLL) patients. LDT is defined as the period of time needed for lymphocytes to double in number the amount found at diagnosis. When the analysis was extended to the whole population, it showed clear differences in the life expectancy of patients with LDT of less than or equal to 12 months (median survival, 36 months; relative death rate [O/E] 1.57) compared with those with LDT of more than 12 months (median survival not yet reached; O/E, 0.37) (P < 0.001). The significance of LDT remained even after adjustments were made for age, sex, lymphocyte count, anemia, and thrombocytopenia. The lack of statistical significance after adjustment for Binet's clinical stage corresponds to the fact that clinical stages are not distributed homogeneously, high LDT being more frequently associated with earlier stages and low LDT with more advanced forms of the disease (P < 0.001). In this study LDT was a useful parameter in predicting disease progression. Patients in Stages A and B and with rapidly increasing lymphocytes counts became worse more frequently (33.3% and 29.1%, respectively, at 12 months after diagnosis) than those with a slow increase (no change in clinical stage at 12 months). It is concluded that since LDT appears to predict the progression of the disease, it is useful in the clinical management of CLL.