Polycystic kidney disease (PKD) is an inherited condition in cats that causes multiple cysts (pockets of fluid) to form in the kidneys. These cysts are present from birth. Initially they are very small, but they grow larger over time and may eventually disrupt kidney function, resulting in kidney failure. While renal cysts are observed with a low incidence in Maine Coons, PKD appears to be a misnomer in this particular breed. In a recent study spanning 8 years, renal cysts were documented by ultrasound in 7 of 187 healthy Maine Coons enrolled in a pre-breeding screening programme. The cysts were mostly single and unilateral (6/7, 85.7%) small (mean 3.6 mm in diameter) and located at corticomedullary junction (4/6, 66.7%), thus different in size, number, and location from those observed in Persian-related breeds. In the same study, not only did all six Maine Coon cats with renal cysts test negative for the PKD1 mutation, proving the disease in these cats to be unrelated to the PKD observed in Persians and related breeds, but gene sequencing of these cats failed to demonstrate any common genetic sequences. 'Maine Coon PKD' thus appears to represent a form of juvenile nephropathy other than AD-PKD.