The cell of origin of B-cell chronic lymphocytic leukemia (B-CLL) is still uncertain. Recent studies have indicated that a fraction of B-CLL displays somatically mutated immunoglobulin variable heavy chain (IgV(H)) genes, which suggests an origin from a post-germinal center (GC) B cell. It has been shown that the 5' noncoding region of the BCL-6 proto-oncogene is affected by mutations in normal GC B-lymphocytes and in lymphoid malignancies displaying GC/post-GC phenotype. To further explore the cellular origin of B-CLL, we have analyzed 34 cases for mutations in the BCL-6 5' noncoding region and in the IgV(H) genes. We found somatically mutated IgV(H) genes in 24 (73%) of 33 samples (average frequency, 6.5 x 10(-2)/bp) and BCL-6 mutations in 8 (24%) of 34 cases (average frequency, 0.14 x 10(-2)/bp in the mutated cases). The occurrence of BCL-6 mutations was restricted to those cases displaying IgV(H) mutations. Analysis of BCL-6 protein expression as a marker of GC phenotype showed that, regardless of the presence of IgV(H) or BCL-6 mutations, B-CLLs express BCL-6 at levels clearly below those found in normal or transformed GC B cells. These results indicate that a subset of B-CLL derives from a cell that has been exposed to the somatic hypermutation mechanism and support the hypothesis that BCL-6 mutations result from the same process that targets immunoglobulin genes.