This study describes an erythema multiforme (EM) in breeding sows, after their mixing in the group housing system. Sows at 30-35 days of gestation showed red and raised skin areas, depression, anorexia, fever, respiratory problems, and increased return to estrus. Blood and nasal samples from diseased sows were examined by quantitative polymerase chain reaction and enzyme-linked immunosorbent assay for respiratory pathogens. Hematological and biochemical analyses were performed on the blood samples. From diseased sows, vaginal swabs for microbiological examinations and samples at slaughterhouse for gross and microscopic examinations were collected. Samples from the complete gestation and lactation feed were examined for mycotoxins. All sampled sows were seropositive for Actinobacillus pleuropneumoniae (App) and porcine reproductive and respiratory syndrome virus (PRRSV). No viremia for PRRSV and porcine circovirus type 2 were detected. All nasal samples were positive for Streptococcus suis, one for Swine Influenza Virus and one for App, Hemophilus parasuis, and S. suis. In all vaginal swabs, Escherichia coli and Streptococcus spp. were detected. Diseased sows had moderate leukocytosis, mild anemia, and thrombocytopenia. No mycotoxins were detected in feed. Histopathological examination revealed increased vascularization of the superficial and middle dermis. EM was likely due to illness caused by viral and bacterial infections. This study suggests that stress caused by the sows' mixing might have triggered the problem.