We examined the possible role of interleukin 10 (IL-10) in the pathogenesis of human chronic B-lymphocytic leukemias (B-CLL). With the use of an in vitro culture technique, we found that IL-10 enhanced the survival of B-CLL cells in a dose-dependent fashion by inhibiting the process of apoptotic cell death. This was demonstrated by transmission electron microscopy and DNA gel electrophoresis. Flow cytometric and immunoblot analyses showed that IL-10 did not significantly upregulate bcl-2 expression as compared with control cultures. B-CLL cells were also found to spontaneously release IL-10 in cultures, and serum IL-10 levels were elevated in five of the eleven B-CLL patients. These findings suggest that IL-10 acts as an autocrine growth factor for B-CLL cells and cytokine-based therapy might be a new approach for the treatment of B-CLL.