Total gastrectomy and chemotherapy with S-1 after surgery were performed in a 50-year-old woman with gastric cancer associated with acquired immunodeficiency syndrome (AIDS). She was given a diagnosis of gastric cancer at the lesser curvature of the body of the stomach, and distal gastrectomy was performed in December 2004. The postoperative course was eventful, with persistent high fever of unknown origin after surgery and infiltrative shadows in the bilateral lung fields showing on CT scan. Polymerase chain reaction (PCR) for pneumocystis carinii on bronchoscopy was positive, serum HIV antibody was positive, HIV-RNA was 2.2 × 10(5) copies/ml, and the serum CD4 lymphocyte level was 25/mm(3) on postoperative day 28. She was given a diagnosis of pneumocystis carinii with AIDS. Pneumocystis carinii and fever improved immediately when ST mixture and highly active antiretroviral therapy (HAART) were performed. After 3 months, the serum CD4 lymphocyte level was elevated to 125/mm(3), and she underwent total gastrectomy because cancer cells at the cut end of the resected stomach were positive microscopically. The postoperative course was uneventful, and she underwent adjuvant chemotherapy with S-1 because the serum CD4 lymphocyte level was 568/mm(3). S-1 therapy was continued for 2 years (each course consisting of 2 weeks of administration followed by 2 weeks off) while performing HAART and monitoring CD4 lymphocyte levels. No side effects such as decreases in white blood cell counts or CD4 lymphocyte levels were seen during S-1 therapy. She is alive and well without recurrence of gastric cancer 5 years after initial gastrectomy.