Objective: To estimate the seroprevalence of Lawsonia intracellularis in finisher pigs and sows in the province of Ontario. Methods: Serum samples from a total of 1061 pigs in 37 Ontario commercial herds were collected and tested using the indirect immunofluorescent antibody test. Differences in within-herd prevalence for type of herd, continuous flow or all-in, all-out management in the finishing barn, and inclusion of in-feed antibiotics were examined. Results: Seroprevalence of L intracellularis was 90% for sows and 56% for finisher pigs. Seventy-three percent of herds were seropositive. Within-herd prevalence was higher in farrow-to-finish farms than in multi-site operations (P < .001), and lower in herds with all-in, all-out rather than continuous-flow management in the finishing barn (P < .01). Implications: Under the conditions of this study, seroprevalence of L intracellularis was high, suggesting that the organism is widespread in the Ontario swine population. More sows than finisher pigs are seropositive. Serologic tests are valuable tools for determining the status of the herd with respect to L intracellularis infection.