The occurrence of bacteria and Ureaplasma urealyticum in the upper urinary tract was studied in 50 patients operated on for renal stones. Cultures were performed on voided urine, pelvic urine obtained during surgery and the stone. The chemical composition of the stones was analysed. Twenty-six stones were of metabolic origin and 24 infection-induced, i.e. composed of struvite and/or carbonate-apatite. Ureaplasma urealyticum was cultured from the upper urinary tract in 1 patient with metabolic stones, and in 7 with infection stones. In 4 of these 7 patients no other urease-producing micro-organism was detected, suggesting that Ureaplasma urealyticum may have been associated with stone formation in these patients.