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 internal medicine departments of 24 institutions between July 1991 and May 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 53 subjects enrolled, 52 (98.1%) were eligible and 47 (88.7%) completed the scheduled treatment (AO/MF group: 23, C/MF group: 24). Although there were significant differences for age and sex between the groups, the Mantel-Haenszel test showed that these unevenly distributed characteristics did not affect the study results. The overall clinical response rates in the completed cases were 30.4% (7/23) in the AO/MF group and 16.7% (4/24) in the C/MF group. In particular, the response rate in the inoperable advanced cases with liver metastases, ascites or distant metastases was 45.5% (5/11) in the AO/MF group versus 16.7% (2/12) in the C/MF group. The treatment-related adverse reactions observed were mainly hematologic and subjective/objective symptoms, such as decreased leukocyte count, hemoglobin level and platelet count, nausea/vomiting, diarrhea, stomatitis, and fever. The differences in the incidence were not significant between the groups. These results show that AO-90 in combination with MF therapy is efficacious in the treatment of patients with gastric cancer.