An emerging swine disease principally involving periweaning piglets was examined. The disease was clinically characterized by lethargy, fever, emaciation, coughing, and severe abdominal breathing, hence colloquially named "Heko-heko" disease. The consistent lesions in affected piglets were diffuse interstitial pneumonia with pronounced type II pneumocytic proliferation, meningoencephalitis, and regression of the lymphoid tissues. The causal virus was isolated in primary porcine lung cell (PLC) cultures from various organs of affected piglets and showed serological relatedness to the European porcine reproductive and respiratory syndrome (PRRS) virus. Numerous virus particles, measured about 49 nm in diameter, were detected in the cytoplasm of alveolar epithelial cells and pulmonary macrophages in PLC cultures infected with the isolate. The condition could be experimentally reproduced in conventional piglets by intranasal inoculation with the isolate and the virus was reisolated from the infected animals.