Porcine reproductive and respiratory syndrome (PRRS) was recorded in Chiba Prefecture in 1989. Twenty-four field cases were examined pathologically. The lungs showed consolidations grayishpink in color in all the cases. Pericarditis was also observed. Histopathologically, the alveolar septa in the lungs were markedly thickened and alveolar spaces were narrow. In the cases with secondary bacterial infection, macrophages and neutrophils infiltrated the alveolar spaces. Lymphoid cells and macrophages also infiltrated the endo-and pericardium, and meninges. A virus was isolated from several field cases and identified as PRRS virus. The hysterectomy-produced colostrums-deprived (HPCD) piglets were inoculated with the virus. In the piglets sacrificed at 7 days after inoculation, interstitial pneumonia which was characterized by a thickening of the alveolar septa was observed. Severe lesions were observed at 28 days after inoculation, and they subsided 42 days after inoculation. The lesions in the piglets in which PRRS virus and Mycoplasma hyorhinis were inoculated, were more severe than those in the piglets in which only PRRS virus was inoculated. As a result of these observations, it was considered that Mycoplasma hyorhinis increased the severity of the lesions in the piglets infected with PRRS virus.