The objective of this study was to estimate the apparent and true prevalence of exposure to Toxoplasma gondii in Ontario finisher pigs. During the study period (2001 to 2004), sera from 6048 pigs were tested with a commercial enzyme-linked immunosorbent assay (ELISA); 103 farms were included 1 to 3 times in the study. True prevalence was estimated using a Bayesian approach. Apparent prevalence at the pig level was 1.59% [95% confidence interval (CI): 0.45, 2.99] in 2001, 0.06% (95% CI: 0.00, 0.46) in 2003, and 0.26% (95% CI: 0.00, 0.82) in 2004. Apparent prevalence at the herd-level was 13.7% (95% CI: 7.5, 22.3) in 2001; 1.25% (95% CI: 0.03, 6.77) in 2003, and 3.75% (95% CI: 0.78, 10.6) in 2004. Similarly, posterior Bayesian estimates of true prevalence at the pig level were 1.7% [95% probability interval (PI): 1.2, 2.2] in 2001, 0.2% (95% PI: 0.04, 0.4) in 2003, and 0.3% (95% PI: 0.1, 0.7) in 2004. At the herd level, posterior estimates of prevalence were 11.6% (95% PI: 7.4, 16.8) in 2001, 0% (95% PI: 0.0, 2.5) in 2003, and 1.2% (95% PI: 0.0, 5.0) in 2004 when a herd cut-point > or = 1 was used. Exposure to T. gondii in finishing pig farms in Ontario appears to be infrequent.