Mice with tail kinks and deformities in their lower extremities were observed in a litter of C.D2-Qa-2+N6F15 mice. A mutant line that exhibits this phenotype in 100% of its offspring was established by subsequent breeding. The abnormalities resembled to some degree those found in a human syndrome termed chronic recurrent multifocal osteomyelitis (CRMO). Accordingly, we name the new mutation chronic multifocal osteomyelitis (cmo). Breeding analysis showed that the defect was determined by a single autosomal recessive gene. Restriction fragment length polymorphism (RFLP) analysis of progeny from a backcross between Mus musculus domesticus (CLA) and C.D2-Qa-(2+)-cmo/cmo indicated that the cmo gene resides on mouse Chromosome 18.