Combination chemotherapy of S-1 and cisplatin has shown promising activity against advanced gastric cancer, but the schedules and dose intensities of S-1 and cisplatin have not been consistent in several clinical trials. We investigated the efficacy and toxicity of 3-weekly S-1/cisplatin chemotherapy as first-line treatment in metastatic or relapsed gastric cancer (MRGC). Forty-six patients with MRGC were prospectively enrolled. S-1 (80 mg/m(2)/day; days 1-14) and cisplatin (60 mg/m(2); day 1) were administrated every 3 weeks. Among 46 patients who received chemotherapy, one achieved a complete response and 21 achieved a partial response, resulting in an overall response rate (RR) of 48%. Thirteen patients (28%) had stable disease and eight patients (17%) had progressive disease. After a median follow-up duration of 48.3 weeks, the median progression-free survival (PFS) and overall survival (OS) were 21.1 weeks and 68.3 weeks, respectively. Patients with good Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0-1 had prolonged PFS and OS compared with patients with ECOG PS of 2. Common hematologic toxicities were anemia (93%), leucopenia (61%), and neutropenia (61%). However, grade 3/4 anemia, leucopenia, and neutropenia developed in only 11, 9, and 24% of patients, respectively. Grade 3/4 non-hematologic toxicities included anorexia (22%), fatigue (13%), nausea (7%), and diarrhea (7%). No treatment-related mortality occurred. Three-weekly S-1/cisplatin chemotherapy was active and well-tolerated in MRGC patients.