In 2003 an outbreak of sudden deaths occurred in 2-3-week-old piglets on a piggery in New South Wales, Australia. There was a marked increase in the birth of stillborn piglets and preweaning losses associated with a multifocal non-suppurative myocarditis with myonecrosis. The aim of this study was to review existing data and to undertake further investigations of specimens from naturally infected pigs to provide evidence to support the hypothesis that Bungowannah virus, a recently recognised pestivirus, causes the porcine myocarditis syndrome (PMC). Sera collected from gilts and sows from affected and unaffected units were tested for Bungowannah virus antibody by a peroxidase-linked assay and Bungowannah virus RNA by qRT-PCR in selected cases. Stillborn piglets from affected and an unaffected unit were also tested for Bungowannah virus antibody and RNA. Body fluid IgG levels and the incidence of myocardial lesions in these stillborn piglets are summarised. Tissue sections from stillborn piglets with myocarditis/myonecrosis were examined for Bungowannah virus RNA by in situ hybridisation. A clear temporal association between the occurrence of PMC on a unit or module and exposure to Bungowannah virus was identified by serological tests in both breeding aged animals and stillborn pigs. In addition, at the individual animal level on affected units, Bungowannah virus RNA was detected in stillborn piglets in large amounts by qRT-PCR and in association with myocardial lesions by in situ hybridisation. The examination of field material from cases of PMC by serology, qRT-PCR and in situ hybridisation provides strong indirect evidence that Bungowannah virus is the causative agent for PMC.