Hematuria and impairment of the renal function was observed in a 50-year-old patient who was receiving penicillin and gentamicin due to a bacterial endocarditis. The symptoms resolved after these antibiotics were changed for vancomycin. The diagnosis of an haemorrhagic cystitis was made through cytoscopy and the possibility of an interstitial nephritis was proposed. These abnormalities are thought to be induced by penicillin. The mechanisms involved would be hypersensitivity reactions and/or direct toxicity. Although interstitial nephritis is a complication rarely observed, its presence combined with haemorrhagic cystitis is quite infrequent. If hematuria appears during an antibiotic treatment with penicillin, one must think of these complications as a possible cause.