Diagnosis of the cause of lower abdominal pain in women may be difficult because appendicitis and pelvic inflammatory disease often present similarly. In a prospective study of 118 women, we found that several criteria are useful in establishing this differential. These include (1) duration of symptoms, (2) the presence of nausea, vomiting or both, (3) a history of venereal disease, (4) cervical motion tenderness, (5) adnexal tenderness, and (6) isolated peritoneal signs in the right lower quadrant. Although no single finding can define the diagnosis, the history and physical findings reported herein provide a number of criteria which, when taken together, will usually allow a confident diagnosis of either appendicitis or pelvic inflammatory disease to be made. Attention to these items can improve precision in diagnosis and lessen the incidence of unnecessary laparotomy, which carries a well-documented complication rate of 10 to 20 percent.