We report on the first case of acute renal failure related to obstructive urinary tract lithiasis involving sulfadiazine crystals in a kidney transplant recipient. This patient had disseminated toxoplasmosis which was treated by sulfadiazine (4 g/day) and pyrimethamine (50 mg/day). In the fourth week of anti-toxoplasmosis therapy, he presented with obstructive acute renal failure: the plasma creatinine level increased from 220 micromol/l to 547 micromol/l. Apercutaneous pyelography was conducted showing the presence of a lithiasis located at the junction between the graft ureter and the bladder. Six days later, he underwent surgery to retrieve an orange-colored, friable stone. Its spectrophotometric analysis confirmed that the stone consisted of N-acetyl sulfadiazine crystals.