A serosurvey on porcine reproductive and respiratory syndrome virus (PRRSV), swine influenza virus (SIV), Aujeszky's disease virus gE protein (ADV gE), porcine parvovirus (PPV) and porcine circovirus type 2 (PCV2) was carried out in Spanish pig herds. The serosurvey consisted of two studies. First, a retrospective study assessed the proportion of seropositive boar, sow and fattening pig herds and their seroprevalences to PRRSV, SIV, ADV gE and PPV from 2003 to 2005 and to PCV2 from 2000 to 2005. Such information was obtained from routine serologic analyses from two veterinary diagnostic laboratory services. Second, a cross-sectional study in sow and fattening pig herds from 44 farms (without vaccination interferences on serologic analyses) was performed to provide information on seroprevalences and co-seropositivity to PRRSV, SIV, ADV gE and PCV2 (PPV was excluded because of widespread vaccination) and to elucidate their relationships with farm characteristics, management and productive parameters. Similar seroprevalences were observed in both studies, although some variations were obtained, probably because of vaccination schedules, number of tested sera, sampling age and regional variations. Percentage of PRRSV and SIV seropositive herds was over 85% for sows, around 80% for fatteners and around 50% for boar studs. The proportion of ADV gE seropositive sow herds decreased from 41% to 30% between 2003 and 2005, whereas such decrease was from 41% to 33% in fattening pig herds and from 13% to 4% in boar studs PCV2 antibodies were widespread as well as those against PPV; in the latter case, if antibodies were elicited by infection and/or vaccination was not assessed. Concurrent presence of PCV2, PRRSV and SIV antibodies was found in 89% and 66% sow and fattening herds, respectively. No statistical associations were obtained between seroprevalences or co-seropositivity and farm characteristics, management or productive parameters.