Peripheral blood lymphocytes from boxer dogs with a history of cutaneous mast cell tumors were cultured for fragile site expression. As in a control group of dogs, cells from these dogs expressed folate-sensitive autosomal and X chromosome fragile sites. Cells from boxer dogs with mast cell tumors expressed the same three common fragile sites on the X chromosome as cells from control dogs. Three folate-sensitive autosomal fragile sites not observed in cells from the control dogs were identified in cells from boxers with mast cell tumors. These included fragile sites near the telomeres of the arms of chromosomes 3 and 4 and a fragile site on the distal half of chromosome 15. Cells from boxers with mast cell tumors showed a greater frequency of fragile site expression than did cells from control dogs, but this observation was attributed to an unintended selection bias for younger boxer dogs without mast cell tumors and older boxer dogs with mast cell tumors and an increased frequency of fragile site expression with increasing age in dogs of the boxer breed.