Abstract Intravenous high-dose cyclophosphamide infusion, usually performed to treat malignant neoplasms, is known to cause water intoxication. Intravenous cyclophosphamide pulse therapy (IVCY) is increasingly being employed for the treatment of rheumatic diseases as well. Recently, water intoxication has been reported to occur even after low-to-moderate doses of IVCY. In the present paper, we describe a case of polyarteritis nodosa in a patient in whom water intoxication developed after IVCY at a moderate dose. Hydration is usually performed to maintain sufficient urine flow to avoid cystitis. Based on our case and a review of the literature, it is recommended that hydration should be carefully performed during IVCY in order to avoid water intoxication, especially when treating elderly patients.