Interleukin-1 (IL-1) beta is a putative mediator of the cancer anorexia/weight loss syndrome, and certain polymorphisms of its gene are thought to be associated with a greater risk of gastric cancer. Do these IL-1 beta genetic polymorphisms predispose patients with gastric and gastroesophageal cancer to the anorexia/weight loss syndrome? This study focused on 44 patients with metastatic gastric and gastroesophageal cancer. All underwent genotyping, completed serial quality-of-life questionnaires germane to appetite, and underwent meticulous serial follow-up. Patients with the IL-1 beta-31 C/T and T/T genotypes were more likely to describe a worse appetite at baseline than were those with the C/C genotype. In addition, patients with the IL-1 beta+3954 C/T and T/T genotypes showed greater improvements in their weight (P = 0.02) and in survival (hazard ratio, 0.3; P = 0.04) over time than did patients with the C/C genotype. These associations occurred independently of tumor response. These preliminary data suggest that certain interleukin-1 beta genetic polymorphisms may modulate the cancer anorexia/weight loss syndrome in patients with metastatic gastric and esophageal cancer. Confirmatory studies are warranted.