Peritonitis originating in the biliary tract is analyzed in 41 patients (1973--1977). Cholelithiasis was the underlying disease in 90% of the cases, and one patient suffered from carcinoma of the biliary tree. Early cholecystectomy was performed in 95% of the patients. Peritonitis was localized in two-thirds and diffuse in one-third of the patients. A perforation was present in 63%. The grade and extent of peritonitis could be correlated with the postoperative course. One-half of the patients with turbid or purulent exudate developed postoperative fever; one fifth showed secondary wound healing. The diffuse form of peritonitis was followed more often by postoperative fever than the localized form. Although the presence or absence of perforation did not influence this parameter in the diffuse form, it did influence cases of localized peritonitis. Only the diffuse forms of peritonitis were followed by secondary wound healing. A peritonitis index was established from age, risk factors, and several preoperative and intraoperative findings. It could also be correlated with the postoperative course. The peritonitis index may classify the disease state of patients with peritonitis and may be found useful in evaluating prognosis at the time of the operation.