Analysis of the experience with scientific studies on patients with secondary intraabdominal infection has revealed that problems of interpretation and comparability between studies exist as they relate to variable diagnostic criteria, unmeasured severity of disease, and unclear outcome measures. A consistent system of definitions has been developed to address these deficiencies. Intraabdominal infection is defined as clinical peritonitis requiring both operative and microbiological confirmation for proof of infection. The APACHE II system is proposed for grading the severity of the infection and for stratification of patient risk of mortality. Mortality and time until death, on one hand, and recovery and time until recovery, on the other, are proposed as the main outcome measures, both being independently and positively defined. It is anticipated that this system of minimum rules will produce studies that can be compared, hence, accelerating knowledge and understanding about intraabdominal infection and its best treatment.