The clinical presentation, course and outcome of Yersinia enterocolitica infection was studied prospectively in 125 children. Enteric forms occurred in 114 children (92 enteritis, 20 pseudoappendicitis, 2 chronic ileitis), of whom 17 also had extramesenteric manifestations; 11 children had one or more extramesenteric forms without enteric disease. Enteritis occurred more frequently in young children whereas serious forms and extramesenteric forms were more common in children older than 6 years of age (P < 0.001). Arthritis was observed in 13 children and extensive lymphadenopathy in 11; 1 child had septicemia with pleurisy, 1 had vasculitis, 1 had cholecystitis and 4 had erythema nodosum. Diagnosis was established by positive culture in 100 (80%) children and by agglutinin test in 11 of 45 (24%), demonstration of circulating specific anti-IgA and anti-IgG to Yersinia outer membrane proteins in 47 of 48 (98%) and detection of antigen in biopsies in 28 of 33 (85%) children. The 2 latter methods were superior to the agglutinin test. Serotype O3 and O9 predominated. The frequency and seriousness of complications may justify the use of antibiotics for Yersinia enteritis in children 6 years of age or older.