New Zealand black (NZB) mice with autoimmune and B lymphoproliferative disease (B-LPD) are a model for human chronic lymphocytic leukemia (CLL). A genomewide linkage scan of the NZB loci associated with lymphoma was conducted in F1 backcrosses of NZB and a control strain, DBA/2. Of 202 mice phenotyped for the presence or absence of LPD, surface maker expression, DNA content, and microsatellite polymorphisms, 74 had disease. The CD5⁺, IgM⁺, B220^dim, hyperdiploid LPD was linked to 3 loci on chromosomes 14, 18, and 19 that are distinct from previously identified autoimmunity-associated loci. The region of synteny with mouse D14mit160 is the human 13q14 region, associated with human CLL, containing microRNAs mir-15a16-1. DNA sequencing of multiple NZB tissues identified a point mutation in the 3′ flanking sequence of the identical microRNA, mir-16-1, and this mutation was not present in other strains, including the nearest neighbor, NZW. Levels of miR-16 were decreased in NZB lymphoid tissue. Exogenous miR-16 delivered to an NZB malignant B-1 cell line resulted in cell-cycle alterations and increased apoptosis. Linkage of the mir-15a/16-1 complex and the development of B-LPD in this spontaneous mouse model suggest that the altered expression of the mir-15a/16-1 is the molecular lesion in CLL.