The gross and microscopic pathologic changes in 70 cases of serologically proven enteric infections with Yersinia pseudotuberculosis are presented. The highest incidence was in young males, and the commonest infecting organism belonged to serologic O-group I. Clinically, the illness resembled acute appendicitis, but the most consistent finding at laparotomy was mesentric lymphadenitis. Surgical specimens examined included 69 mesenteric lymph nodes, 18 appendices, five terminal ileums, and two ascending colons. Histologically, four stages of the disease were identified, leading to the formation of characteristic granulomas with central necrosis and microabscess formation. Ulceration of the intestinal and appendicular mucosa may occur. The illness usually runs a benign course, and antibiotic treatment is rarely necessary. The pathogenesis and differential diagnosis are discussed with reference to the current literature.