Under-reporting of infectious gastrointestinal illness (IGI) in British Columbia, Canada was calculated using simulation modelling, accounting for the uncertainty and variability of input parameters. Factors affecting under-reporting were assessed during a cross-sectional randomized telephone survey. For every case of IGI reported to the province, a mean of 347 community cases occurred (5th and 95th percentile estimates ranged from 181 to 611 community cases, respectively). Vomiting [odds ratio (OR) 2.15, 95% confidence interval (CI) 1.03-4.49] and antibiotic use in the previous 28 days (OR 3.59, 95% CI 1.17-10.97) significantly predicted health-care visits in a logistic regression model. In bivariate analyses, physicians were significantly less likely to request stool samples from patients with vomiting (RR 0.09, 95% CI 0.01-0.65) and patients of North American as opposed to non-North American cultural groups (RR 0.38, 95% CI 0.15-0.96). Physicians were more likely to request stool samples from older patients (P=0.003), patients with fewer household members (P=0.002) and those who reported anti-diarrhoeal use following illness (RR 3.33, 95% CI 1.32-8.45). People with symptoms of vomiting were under-represented in provincial communicable disease statistics. Differential degrees of under-reporting must be understood before biased surveillance data can be adjusted.