A review of the chromosome findings in 25 male tortoiseshell or calico (T-C) cats showed a variety of aneuploidy, polyploidy, mosaicism, and chimerism. An XXY-complement was included in the chromosome makeup of 16 of the 25 cats. Almost all of these cats were sterile. Testicular pathologic changes, when recorded, appeared comparable with that of human XXY Klinefelter's syndrome. The findings in 2 male T-C cats were presented as representative models of XXY Klinefelter's syndrome in man. Other findings in human Klinefelter's syndrome have not been looked for in the feline models, but a plan for this is underway. A review also was made of the historical steps leading to our present understanding of the Klinefelter syndrome including the role of Felis catus. An explanation of cat coat-color genetics clarified why "black" and sex-linked "orange" coloration can appear together normally in XX females and in rare males with 2 different X chromosomes. Thus, male T-C cats were only convenient indicators of underlying sex-chromosome aberrations which were occurring completely independent of coloration.