In 1987, a nationwide outbreak of Salmonella typhimurium O:4-12 infection traced to contaminated chocolate bars occurred in Norway. In the 5 years after the outbreak, elevated numbers of sporadic cases caused by the epidemic strain of Salmonella were detected, followed by a decline in subsequent years. To characterize the epidemiology of this infection, the authors analyzed information concerning all sporadic cases reported in Norway from 1966 to 1996. Of the 153 patients infected by the outbreak strain, 43% were less than 5 years of age, and only three persons had acquired the infection abroad. In contrast, 46% of the cases attributable to other S. typhimurium O:4-12 variants and 90% of the total number of Salmonella infections were related to foreign travel. A distinct seasonality was observed: 76% of the cases appeared between January and April. At the same time of year, the epidemic strain was regularly encountered as the etiologic agent of fatal salmonellosis among wild passerine birds, suggesting an epidemiologic link between the avian and human cases. The strain was rarely isolated from other sources. From 1990 to 1992, the authors conducted a prospective case-control study of sporadic indigenous infections to identify risk factors and obtain guidance for preventive efforts. Forty-one case-patients, each matched by age, sex, and geographic area with two population controls, were enrolled. In conditional logistic regression analysis, the following environmental factors were independently related to an increased risk of infection: drinking untreated water, having direct contact with wild birds or their droppings, and eating snow, sand, or soil. Cases were also more likely than controls to report having antecedent or concurrent medical disorders. Forty-six percent of the study patients were hospitalized for their salmonellosis.