B-cell chronic lymphocytic leukemia (B-CLL) is a heterogeneous disease that may exhibit familial clustering. We examined the cytogenetic, immunophenotypic, and VH gene usage characteristics of a family with B-CLL affecting 7 members in 3 generations. Interphase fluorescence in situ hybridization studies identified an acquired deletion of chromosome 13q14 in the leukemic cells of 6 affected members, accompanied by deletion 14q32 or trisomy 12 in 2 cases. VH gene analysis demonstrated clonal rearrangements of the VH3 gene family in 5 cases and of VH2 genes in 1 case. All 6 cases were mutated in VH2 or VH3. Two cases had a second VH1 family gene rearrangement that was unmutated. Flow cytometry performed on 5 cases showed the typical B-CLL immunophenotype; all were CD38-, but 3 expressed ZAP-70. Our findings support previous observations that familial and sporadic B-CLL cases are biologically similar and suggest that familial clusters will be useful for studying pathogenetic events in B-CLL.