A systemic necrotizing vasculitis of unknown etiopathogenesis may be termed juvenile polyarteritis syndrome (JPS). The syndrome has been recognized primarily in young Beagles used for toxicologic studies. We studied 9 young Beagles with JPS. Affected dogs had fever (40 to 41.5 C), anorexia, and signs of pain in the cervical area. They had a characteristic hunched stance, and were unwilling to move. Laboratory abnormalities in all dogs included nonregenerative anemia, hypoalbuminemia, and leukocytosis characterized by a mature neutrophilia. Analysis of CSF revealed a moderate to severe neutrophilic pleocytosis and a mildly high protein concentration in most dogs. Signs of disease resolved rapidly with high doses (2.2 mg/kg of body weight, PO) of prednisone. If untreated, clinical signs and laboratory abnormalities had a remitting and relapsing course in most dogs. Findings at necropsy included necrotizing arteritis with fibrinoid necrosis, periarteritis, thrombosis, and intimal proliferation that most frequently affected small- to medium-sized vessels in the cervical spinal cord, mediastinum, and heart. An immune-mediated pathogenesis for this disease is suspected.