A pathological study on 5 of 21 cats affected naturally with systemic calcinosis was performed. The animals ranged in age from 1 to 9 years. Hematology and serum chemistry analyses showed the elevated values of phosphorus, blood urea nitrogen and serum creatinine. X-ray examination disclosed the increased density of systemic bones. Histologically, marked calcification was present at the vascular walls of almost all the organs including the lungs, trachea, kidneys, heart, aorta, alimentary tracts, choroid plexus and bones. In the lungs, kidneys and stomach, the calcified lesions were associated with deposition of oxalate crystals. Serum chemistry showed more elevated values of 25-hydroxycholecalciferol (vitamin D) of the affected cats than the normal level. Retrospective examination revealed that these cats had been fed the commercial pet foods containing a large amount of vitamin D (6,370 IU/100 g diet) from their young age, and its value was about ten times as much as that of the control food (680 IU/100 g diet). Pathological changes found in the cats from the experimental vitamin D3 toxicosis were similar to those in the natural cases. In addition, tissue levels of calcium, phosphorous and zinc in the lungs and kidneys were markedly elevated in both natural and vitamin D-intoxicated cases. These findings suggest that long-term feeding of the pet food containing excessive vitamin D was responsible for the outbreak of the systemic calcinosis in the cats.