OBJECTIVE We evaluated the sensitivity of PCR on oral fluids in detecting influenza virus in vaccinated and non-vaccinated pigs. METHODS Three-week-old influenza-free pigs were divided into three groups: (i) control, non-vaccinated, (ii) vaccinated with a commercial, heterologous vaccine, and (iii) vaccinated with an experimental, homologous vaccine. After vaccination, an influenza-infected pig was placed in contact with each of the groups. Individual nasal swabs and pen oral fluids were collected daily. Viral RNA was tested for the presence of influenza by RRT-PCR and virus isolation attempted from oral fluids. A pen was considered positive if at least one nasal swab was positive. RESULTS Based on nasal swab results, 43·8% of pens were detected positive but only 35% based on oral fluids. Overall sensitivity of oral fluids was 80%, and virus was isolated from 51% of RRT-PCR-positive oral fluids. The kappa coefficient for agreement (ĸ) between oral fluids and nasal swabs was 0·82. Among groups, ĸ was 1 (95% CI, 1-1), 0·74 (95% CI, 0·55-0·92), and 0·76 (95% CI, 0·5-1) for control, heterologous, and homologous-vaccinated groups, respectively. There was less agreement when within pen prevalence was 10% or less. Probability of detecting influenza virus in oral fluids was 99% when within pen prevalence was higher than 18% and decreased to 69% when prevalence was 9%. CONCLUSIONS Results indicated that pen-based collection of oral fluids is a sensitive method to detect influenza even when within pen prevalence is low and when pigs have been vaccinated and highlight the potential use of oral fluids for influenza surveillance.