Porcine necrotic ear syndrome is a disease of swine characterized by large erosive lesions at the margin of the pinna(e). The gross and microscopic characteristics of the lesions were studied in 38 pigs selected from eight affected swine herds. The progression of the lesions was examined in a herd of 174 weaned pigs in a total confinement nursery. The lesions began as a superficial vesicular dermatitis associated with superficial auricular trauma and progressed to become exudative and encrusted. Localized lesions slowly healed or sporadically progressed to deep necrotic ulcers. The early lesions resembled the epidermal changes produced by Staphylococcus hyicus. Deep ulceration and necrosis was attributed to the invasion of streptococci into the dermis resulting in cellulitis, vasculitis, thrombosis, ischemia, and necrosis.