Cyclophosphamide belongs to alkylating drugs and it has been widely used in rheumatology. Polymyositis is one of the diseases that may require treatment with cyclophosphamide. The administration of the drug can be disadvantageous due to its possible side effects. We describe a case of a 61-year-old patient with polymyositis, treated with cyclophosphamide which had good therapeutic effect and was initially well-tolerated. After the eighth intravenous cyclophosphamide infusion, lower gastrointestinal bleeding occurred. Colonoscopy revealed numerous ulcerations of the distal colon and the histopathologic examination showed non-specific colitis. Hemorrhagic colitis after cyclophosphamide was suspected and the drug was stopped. The symptoms resolved within ten days after treatment with hydrocortisone enema and did not recur during further 2-year follow-up. Cyclophosphamide may cause serious side effects and beside these which are frequent and well-known, rare adverse events including hemorrhagic colitis should also be taken into account.