The prevalence of chromosome abnormalities is increased in infertile men, the incidence of a chromosomal factor being estimated to be about 8%. We report two brothers, a 38-year-old man with 10 years' primary infertility and severe oligospermia, but otherwise healthy, and a 35-year-old man with primary infertility and a history of mumps during puberty. Semen and karyotype analysis, and investigation of Y-chromosome microdeletions were performed. An apparently unique reciprocal translocation t(6;12)(q23;q24.3) was found in both infertile brothers. Semen analyses showed severe oligospermia. No Y-chromosome microdeletions were found. These two cases support the relationship between both environmental and chromosomal abnormalities, combined or separated, with male infertility. Investigation of genetic alterations in infertile males has to be performed prior to performing any assisted reproduction technique.