Difluorodeoxycytidine (dFdC) demonstrated broad spectrum activity in preclinical models. A phase 1 study utilizing twice weekly injections was conducted in 50 eligible and evaluable patients. Twenty-nine patients received drug by 30 minute infusion at doses of 5-90 mg/m2 and 22, by 5 minute bolus at 30-150 mg/m2. The primary dose limiting toxicities were marrow suppression and flu-like symptomatology. Thrombocytopenia was dose limiting at 75 mg/m2 on the infusion schedule and 150 mg/m2 on the 5 minute schedule. Flu-like symptoms with fever, rigors and malaise occurred the day of injection in many patients. One patient with renal cell carcinoma attained a partial response. Evaluation of the drug's efficacy and schedule dependency continue.