Among 60 patients with chronic lymphocytic leukemia, higher in vitro uptake of tritiated (3H) thymidine by leukocytes of a standard volume of peripheral blood was associated with a higher lymphocyte count, a more advanced stage, greater frequency of functional impairment and shorter survival. Appropriate analyses demonstrated that leukocyte thymidine uptake correlated with survival independently of these other disease features. Relative thymidine uptake (radioactivity per 10(3) lymphocytes) did not prove to be a useful prognostic parameter. Among 33 patients not receiving antileukemic therapy at the time of study, 15 of 17 (88 per cent) of those with higher thymidine uptake values, but only 3 of 16 (19 per cent) of those with lower values, were treated during a median follow-up period of four and a half years (p less than 0.001). Seven of the former group, but none of the latter group, died during the first three years of follow-up (p less than 0.01). We conclude that thymidine uptake by circulating leukocytes constitutes a relatively accurate index of the proliferating leukemic cell mass in this disease and provides useful prognostic information.