Between May 1990 and April 1994, eleven patients with metastatic renal cell carcinoma received a combination therapy with interferon-alpha (IFN alpha) and 5-fluorouracil (5FU). IFN was administered intramuscularly six or ten million units three times per week for 4 weeks and 300 mg/m2 of 5FU was administered by continuous intravenous infusion daily for 4 weeks. Of 8 evaluable patients, two had a partial response (25%) two had a minor response (25%), and two had a stable disease (25%). The common side effects of the regimen were flu-like symptoms (91%), mucositis (64%) and leukopenia (75%). Three patients refused this therapy because of severe mucositis or stomatitis. Although the combination of IFN and 5FU in patients with metastatic renal cell carcinoma had some efficacy, this regimen had severe toxicity especially for the gastrointestinal (GI) tract. None of the patients could be administered the initially scheduled dosage of 500 mg/m2 of 5FU. The dose limiting factor of this regimen is considered to be GI symptoms.