B cell chronic lymphocytic leukemia (CLL) CD38 expression is variable and may predict outcome. Inducible nitric oxide synthase (NOS2) expression regulates CLL cell apoptosis. IL-4 and IFN-gamma regulate B cell CD38 expression and NOS2 expression. We compared IL-4 and IFN-gamma serum levels between CLL patients and normal individuals, and determined whether serum IL-4 and IFN-gamma levels correlated with CLL cell CD38 expression and NOS enzyme activity. IL-4 levels, but not IFN-gamma levels, differed between normal individuals and CLL patients. Furthermore, there was an association of IL-4 levels, but not IFN-gamma levels, with CD38 and NOS2 expression in CLL patients.