BACKGROUND: The association between asthma and anaphylaxis remains poorly understood. OBJECTIVE: To ascertain, in a managed care organization in northern California, the association of asthma and asthma severity with future risk of anaphylactic shock and other selected allergy diagnoses. METHODS: Using electronic data and validated algorithms, we assembled a cohort of 526,406 patients who met the criteria for asthma between 1996 and 2006 and a referent cohort (with no utilization for asthma) individually matched on age, sex, and race/ethnicity. In each cohort, 54% of patients were female and 55% were white; their mean (SD) age was 24 (20) years. The main outcome measures were anaphylactic shock (caused by an adverse food reaction, caused by serum, or other/idiopathic), allergic urticaria, anaphylaxis after sting(s), and angioedema. RESULTS: The incidence of anaphylactic shock was 109.0 per 100,000 person-years in the asthma cohort and 19.9 per 100,000 person-years in the referent cohort. After adjustment for age, sex, race/ethnicity, comorbidities, and immunotherapy, asthma was associated with a 5.2-fold (95% confidence interval, 4.7- to 5.6-fold) increased hazard of anaphylactic shock. Asthma was also significantly associated with an increased risk of the 3 selected allergy diagnoses, with hazard ratios of 1.4 to 1.9. A significant trend by severity of asthma was apparent for food-related and other/idiopathic anaphylactic shock and for anaphylaxis after sting(s). CONCLUSIONS: In this insured population, asthma was prospectively associated with increased risk of anaphylactic shock and other allergy diagnoses. However, the effect of asthma severity was not consistent across outcome measures.