A previously unreported urolith is presented. The concrement was found in the bladder of four Persian cats and one collie dog. It contained potassium, magnesium and phosphorus, all determined by chemical analysis. The infrared spectrum of the X-ray-dense urolith displayed absorption bands in the phosphate region. A single absorption maximum at 930 cm(-1) indicated the presence of inorganic pyrophosphate. A single crystal structure determination of the urolith has been performed. The results obtained are consistent with the formula [K(1.0)Mg(1.5)](4+)[P(2)O(7)](4-.)(H(2)O)(5), which corresponds to 11.5% K, 10.7% Mg and 18.2% P. The structure contains negatively charged slabs of composition MgP(2)O(7) small middle dot(H(2)O)(2)(2-)alternating with positively charged, hydrated Mg(2+) and K(+) ions. The observed P(2)O(7) is that of a distorted eclipsed syn conformation, which is the most common geometry in the solid state according to a statistical analysis of known structural data. A simple numerical descriptor has been derived, classifying observed X(2)O(7) conformations. It is suggested that some genetic enzymatic dysfunction of pyrophosphate-hydrolysing alkaline phosphatase might cause the formation of this unusual urolith.