Health administrative databases can be used to track disease incidence, outcomes and care quality. Case validation is necessary to ensure accurate disease ascertainment using these databases. Disease-specific codes and algorithms (combinations of disease-specific health services codes) have been validated in various regions to identify adult-onset IBD. We previously validated an algorithm to identify pediatric-onset IBD and create the Ontario Crohn's and Colitis Cohort (OCCC), a population-based surveillance cohort. In this study, we aimed to validate adult-onset IBD identification algorithms to expand the OCCC to adults. Two cohorts of patients were used to validate algorithms: 1) Ottawa Hospital Algorithm Development Cohort: An electronic database (Ottawa Hospital Data Warehouse) of outpatient visits, hospitalizations, pathology and radiology reports was searched. The charts of patients with either a diagnostic ICD code for Crohn's or UC, or a referral to IBD, Crohn's or UC in a keyword search of either a histology or radiology report were extracted (n = 5847). Patients >18 years seen at the Ottawa Hospital from fiscal years (FY) 2002-2005 were classified as incident IBD (n = 554), prevalent IBD (n = 1193) or non-IBD (n = 3330). 2) Ontario Algorithm Validation Cohort: The charts from 5 community practices (family medicine and gastroenterology) and 3 tertiary care centers across Ontario, comprising the practices of >30 physicians, were extracted to classify IBD patients diagnosed FY2001-2005 (n = 464) and non-IBD patients (n = 1051). We linked to health administrative databases from FY1991-2010 and compared the accuracy of various algorithms with various lengths of time to qualify. In addition, their latest diagnosis (Crohn's or UC) was determined from charts to validate an algorithm to distinguish patients with Crohn's from those with UC. Over 5000 algorithms were tested. Table 1 details the diagnostic accuracies of published algorithms. One health care contact was not adequate to identify patients with IBD. The Manitoba algorithm attained the lowest false-positive rate, while maintaining sensitivity. The algorithms functioned variably by age, with diagnostic accuracy lower in patients with onset >65 years (e.g. Ottawa Hospital Algorithm Development Cohort for the Manitoba algorithm: Sens 59.3%, Spec 98.2%, PPV 58.2%, NPV 98.3%). Having 5 of the last 9 physician billing codes for Crohn's or UC accurately distinguished IBD subtypes (accuracy 91.1%), while having 4 of the last 7 codes (accuracy 90.7%) and 5 of the last 8 codes (accuracy 90.2%) also functioned adequately. Patients with adult-onset IBD can be accurately identified from within health administrative data. The previously validated algorithms from Manitoba were the most accurate. Further work will explore other algorithms, particularly in patient subgroups such as the elderly. The OCCC will be expanded to include adult patients, creating a large, population-based surveillance cohort of all patients with IBD in Ontario, Canada.