Pelvic inflammatory disease is the most common serious complication of sexually transmitted infections caused by Neisseria gonorrhoeae and Chlamydia trachomatis. Initial episodes of sexually transmitted acute PID occur most frequently in the 20 to 24 year age group, while sequelae such as infertility, ectopic pregnancy, and chronic PID are most common among 25- to 34-year-old women. The apparent chronicity of the disease process suggests that the epidemic of sexually transmitted disease, which began in the mid-1960s, could be followed by an epidemic of PID and its sequelae. More than 850,000 episodes of PID occur annually, requiring more than 212,000 hospital admissions, 115,000 surgical procedures, and 2,500,000 physician visits. Ectopic pregnancies in the United States tripled from 1967 to 1977. Direct and indirect costs for PID and PID-associated ectopic pregnancy exceeded $1.25 billion in 1979. If PID and ectopic pregnancy rates continue unabated, by the year 2000 there will have been more than one episode of PID and three related physician visits for every two women who reached reproductive age in 1970. Fifteen percent will require hospitalization, more than 3% will experience an ectopic pregnancy, and more than 10% will involuntarily become sterile because of PID.