During October 1988 through January 1990, a study of sporadic Yersinia enterocolitica infections was done in the Oslo region to assess the clinical impact and risk factors for this disease. Sixty-seven case-patients (mean age, 23.4 years) and 132 population-based age- and sex-matched controls were enrolled. Among patients who were well when interviewed, illness lasted a mean of 20 days, but 10% of the others remained symptomatic a year later. Bloody diarrhea occurred only in persons less than 18 years old (P = .002); joint pain was more common in adults (P = .001). Prolonged carriage was found in 47% of patients after resolution of symptoms. Patients were less likely to shed the organism after antimicrobial treatment (relative risk, 0.3; P = .003). Case-patients were more likely than controls to have antecedent enteric illness (odds ratio, 8.2; P less than .001). Y. enterocolitica infection in Norway is notable for its severity and chronicity. Postsymptomatic shedding, which occurs commonly, may be reduced by antimicrobial treatment.