Abstract Two patients, a 58-year-old man and a 49-year-old woman, received metronidazole 0.4 g thrice daily for prevention and treatment of oral infection. They presented with gross hematuria on days 2 and 3 after drug administration, respectively, followed by lumbago, urgency of urination, urinary frequency, painful urination, and other symptoms. Routine urine tests revealed RBC（++）in the male patient and numerous RBC in the female patient. Metronidazole was discontinued and switched to combination treatment with cefalexin and erythromycin ethylsuccinate. Their symptoms improved gradually and then disappeared.