BACKGROUND: Previous studies provide evidence that there is a greater frequency of autoimmune diseases among patients with psoriasis than in the general population. OBJECTIVE: This study examined the association between psoriasis and 21 common autoimmune diseases, specified a priori. METHODS: A retrospective cohort study was conducted among persons who were members of Kaiser Permanente Southern California from 2004 to 2011. A total of 25,341 patients with 2 or more diagnosis codes for any psoriatic disease were evaluated. Five persons not meeting this case definition were matched to each psoriatic patient based on age, sex, and length of enrollment. RESULTS: Patients with psoriasis were more likely to have at least 1 other autoimmune disease (odds ratio [OR] 1.6; 95% confidence interval [CI] 1.5-1.7) and to have at least 2 other autoimmune diseases (1.9; 95% CI 1.6-2.4). Of the 17 conditions evaluated, associations with 14 were found to be statistically significant. The strongest association was with rheumatoid arthritis (3.6; 95% CI 3.4-3.9). LIMITATIONS: Patients with autoimmune conditions are likely to have a greater number of health care encounters, which may result in overascertainment and misascertainment of immune-mediated conditions, although the patients included in the study averaged 5.2 years of observation and the comparison subjects were matched on length of enrollment. CONCLUSIONS: The study suggests a genetic or environmental cause common across autoimmune diseases. Further investigation of individuals with multiple autoimmune diseases may yield important clues about the origin and pathogenesis of the disease.