Although several outbreaks of Trichinella infection among humans have occurred in northwestern Vietnam, no information is available on the circulation of Trichinella among domestic pigs. The objective of the present study was to estimate the seroprevalence of anti-Trichinella IgG in free-roaming pigs (Sus scrofa) in the Son La province of northwestern Vietnam, where a human outbreak of trichinellosis occurred in June 2008. Serum samples were collected from free-roaming pigs of four communes of the Bac Yen district (Son La province) and tested for Trichinella antibodies with a commercial ELISA kit using excretory/secretory antigens. Of 1035 pigs from which serum samples were collected, 206 were positive (19.9%). There was a significant difference in the prevalence among communes (chi(2)=22.87, 3 d.f., p<0.0001). Muscle samples from 76 serologically positive pigs were tested by artificial digestion. Trichinella larvae were detected in 11 (14.5%) of them. The larvae were identified by multiplex PCR as Trichinella spiralis. This study provides the first data on the circulation of T. spiralis in domestic pigs reared in Vietnam, and the results are useful for evaluating the risk of infection for humans. The results indicate that pigs act as a reservoir and play an important role in the maintenance of the domestic cycle of T. spiralis in northwestern Vietnam.