The syndrome of inappropriate secretion of antidiuretic hormone (SIADH) is often drug-induced. We describe the first case, to our knowledge, of SIADH consecutive to ciprofloxacin administration. On two occasions, a 70-year-old patient, known for post-actinic bladder with repeated episodes of hematuria, was given ciprofloxacin and developed severe hyponatremia with seizures. On both occasions, the patient's condition improved rapidly after withdrawal of the drug. As all organic diseases likely to cause SIADH were excluded, and because the drug rechallenge was positive, we suggest that ciprofloxacin be added to the list of drugs prone to induce SIADH.