Chronic lymphocytic leukemia (CLL) is typically an indolent lympho-proliferative disorder. Its clinical course is notable for marked heterogeneity, but in a subset of patients, the disease pursues a relatively rapid clinical course. To identify patients that may have aggressive disease, the growth fraction as determined by Ki-67 proliferation marker, DNA and RNA content, and IL-2 receptor expression were determined in 46 patients with CLL by flow cytometry. Our results indicate a significant statistical correlation between Ki-67 positivity and IL-2 expression in B-CLL cells. No correlation between the proliferative activity or RNA content and IL-2 expression was found. Our data indicate that measurement of both IL-2 receptor and Ki-67 expression in B-CLL can identify a subset of patients with a high risk of rapid clinical progression.