A personal history of autoimmune diseases is associated with an increased incidence of gastric cancer, but whether they share familial susceptibility is still unknown. The contribution of shared environmental or genetic factors toward the observed familial aggregation has not been determined. We used a few Swedish registers, including the Swedish Multigeneration Register and the Cancer Register, to examine the familial risk of gastric cancer among individuals with a family history of a set of autoimmune diseases. Standardized incidence ratios were used to calculate the relative risk. The overall risk of gastric cancer was 1.22 (95% confidence interval: 1.14-1.30) among individuals with a sibling affected with any of the 33 autoimmune diseases. For specific disease, siblings of individuals with Crohn's diseases, diabetes type 1, Graves'/hyperthyroidism, myasthenia gravis, psoriasis, rheumatoid arthritis, sarcoidosis, and uncreative colitis showed an association with an increased incidence of gastric cancer, with a standardized incidence ratio ranging between 1.17 and 1.64. Familial aggregation was found only for corpus cancer. No association was observed between spouses. Gastric cancer, mainly corpus cancer, shares familial susceptibility with a few autoimmune diseases, suggesting that shared genetic polymorphisms may contribute toward both Helicobacter pylori infection and autoimmune diseases.