A 59-yr-old male alcoholic with bilateral nephrocalcinosis and upper urinary tract stones presented with fever, acute abdominal signs and ascites. Laparotomy revealed the presence of 1.5 litres of ascitic fluid and confirmed right-sided acute pyelonephritis. Culture of urine from the renal pelvis obtained during surgery was positive for Mycoplasma hominis. Initial therapy with cefuroxime failed and doxycycline was later initiated when culture was positive for Mycoplasma hominis, with definite clinical improvement. This is an unusual case of acute pyelonephritis with peritoneal signs and ascites due to Mycoplasma hominis in an elderly male who had no recent history of urinary tract instrumentation.