The prevalence of infections with H1N1- and H3N2-influenza viruses, porcine respiratory coronavirus (PRCV), transmissible gastroenteritis virus (TGEV) and porcine epidemic diarrhoea virus (PEDV) in feeder pigs shortly after their entry into fattening units was examined. Ten groups of pigs with acute respiratory disease during the months September to October 1991 and seven groups of pigs with acute diarrhoea during the months February to March 1992 were investigated. On arrival in the fattening herds, more of the pigs were negative for antibodies against H1N1-influenza virus and against PRCV during September to October (61 and 50 per cent, respectively) than in February to March (51 and 34 per cent, respectively). There was serological evidence of a triple infection with PRCV and both influenza viruses in seven of the 17 groups; dual infections with PRCV and H1N1-influenza virus occurred in nine groups and with H1N1- and H3N2-influenza viruses in one group. Seroconversion against TGEV was not detected in any of the 17 groups, but seven of them had seroconverted to PEDV. Multiple infections with PRCV and either one or both of the influenza viruses were thus very common shortly after the introduction of feeder pigs into the fattening herds. There was no association between the type and/or multiplicity of these infections and respiratory disease, but infections with PEDV were clearly associated with outbreaks of diarrhoea.