The results of recent preclinical and clinical studies suggest that AO-90, a methionine-free intravenous amino acid solution (7.43%), potentiates the antitumor effect of 5-fluorouracil (5-FU). In the present multi-center, randomized, controlled study conducted at the surgery departments of 53 institutions between July 1991 and March 1993, patients with advanced gastric cancer were randomly allocated to receive either AO-90 (500-750 mL/day, AO/MF group) or Amiparen, a commercial intravenous amino acid solution (600-800 mL/day, C/MF group) by total parenteral nutrition for 14 days. Both groups received MF therapy which consisted of a continuous infusion of 5-FU at 350 mg/m2/day for 14 days and an i.v. push of mitomycin C 7 mg/m2 on days 7 and 14 (one course). Additional treatment courses were initiated after a withdrawal period when appropriate. Of the 138 subjects enrolled, 129 (93.5%) were eligible and 119 (86.2%) completed the scheduled treatment (AO/MF group: 57, C/MF group: 62). The overall clinical response rates in the completed cases were 26.3% (15/57) in the AO/MF group and 8.1% (5/62) in the C/MF group, and the difference between the groups was significant (p = 0.015). In particular, the response rate in the postoperative recurrent patients with measurable lesions was 42.9% (12/28) in the AO/MF group versus 12.0% (3/25) in the C/MF group (p = 0.016). Further, in the patients who were previously treated with fluoropyrimidine drugs, 29.0% (9/31) responded to the AO/MF therapy versus 8.6% (3/35) in the C/MF group (p = 0.053). The treatment-related adverse reactions observed were mainly hematologic and subjective/objective symptoms, such as decreased leukocyte count and hemoglobin level, nausea/vomiting and stomatitis. The differences in the incidence were not significant between the groups. Based on these results, AO-90 in the MF regimen appears to be effective in the treatment of patients with advanced gastric cancer by significantly potentiating the effects of 5-FU.