Actinobacillus suis was isolated from tissues of 39 pigs, 2 porcine lungs, and 1 uterine swab submitted for diagnostic evaluation from 24 farms in southwestern Ontario between 1985 and 1988. These isolates represented a gradually increasing incidence of herd outbreaks caused by A. suis in southwestern Ontario. The outbreaks were typified by sudden death in suckling or recently weaned pigs; 87% of the affected pigs examined at the laboratory were between two and 28 days old. Petechial to ecchymotic hemorrhages in the thoracic and abdominal organs accompanied by serofibrinous exudates in both cavities were the most common gross lesions. The lesions were characterized histologically by bacterial thromboembolism and necrosis randomly scattered in thoracic and abdominal organs. Occasionally, bacterial thromboemboli were surrounded by centrifugally radiating, eosinophilic, club-like colonies. Diffuse necrohemorrhagic myocarditis that was more severe in the atria, and diffuse subacute meningoencephalitis, were less frequent but distinctive lesions. Multiple litters were affected in most herd outbreaks, and mortality often approached 50% in affected litters. Although the A. suis organism was susceptible to nearly every antibiotic against which it was tested, the suddenness of herd outbreaks precluded attempts at treatment.