We measured telomerase activity (TA) in bone marrow samples from 214 patients with CLL and correlated it with patients' characteristics and survival. In >50% of cases (126/214; 59%) no detectable TA was found. There was no difference in TA between previously treated (n = 153) and untreated (n = 61) patients (P = 0.4), or patients with various Rai (0-IV) stages (P = 0.85). TA correlated significantly with white blood cell and lymphocyte count (P = 0.02 and 0.01, respectively) but not with bone marrow cellularity, beta2-microglobulin (beta2M), or other patient characteristics. Patients who had no TA had slightly lower beta2M and lower lymphocyte counts (P = 0.5 and 0.04, respectively) as compared with patients with detectable TA. However, there was no correlation between TA and survival. This data suggests that TA may not play a significant role in the clinical behavior of CLL.