The mutational status of tumor immunoglobulin V(H) genes is providing a powerful prognostic marker for chronic lymphocytic leukemia (CLL), with patients having tumors expressing unmutated V(H) genes being in a less favorable subset. However, the biologic differences correlating with V(H) gene status that could determine the clinical course of the disease are unknown. Here we show that differing responses to IgM ligation are closely associated with V(H) gene status. Specifically, 80% of cases with unmutated V(H) genes showed increased global tyrosine phosphorylation following IgM ligation, whereas only 20% of samples with mutated V(H) genes responded (P =.0002). There was also an association between response to IgM ligation and expression of CD38 (P =.015). The Syk kinase, critical for transducing B-cell receptor (BCR)- derived signals, was constitutively present in all CLL samples, and there was a perfect association between global phosphorylation and induction of phosphorylation/activation of Syk. Nonresponsiveness to anti-IgM could be circumvented by ligation of IgD (10 of 15 samples tested) or the BCR-associated molecule CD79alpha (12 of 15 samples tested). These results suggest that multiple mechanisms underlie nonresponsiveness to anti-IgM in CLL and that retained responsiveness to anti-IgM contributes to the poor prognosis associated with the unmutated subset of CLL. The prognostic power of the in vitro response to IgM ligation remains to be determined in a large series, but the simple technology involved may present an alternative or additional test for predicting clinical course.