OBJECTIVE To estimate direct medical costs and average lifetime cost per case of pelvic inflammatory disease (PID). METHODS We estimated the direct medical expenditures for PID and its three major sequelae (chronic pelvic pain, ectopic pregnancy, and infertility) and determined the average lifetime cost of a case of PID and its sequelae. We analyzed 3 years of claims data of privately insured individuals to determine costs, and 3 years of national survey data to determine number of cases of PID, chronic pelvic pain, and ectopic pregnancy. We developed a probability model to determine the average lifetime cost of a case of PID. RESULTS Direct medical expenditures for PID and its sequelae were estimated at $1.88 billion in 1998: $1.06 billion for PID, $166 million for chronic pelvic pain, $295 million for ectopic pregnancy, and $360 million for infertility associated with PID. The expected lifetime cost of a case of PID was $1167 in 1998 dollars. The majority of those costs ($843 per case) represent care for acute PID rather than diagnosis and treatment of sequelae. Approximately 73% of cases will not accrue costs beyond the treatment of acute PID. CONCLUSION The direct medical cost of PID is still substantial. The majority of PID related costs are incurred in the treatment of acute PID. Because most PID-related costs arise in the first year from treatment of acute PID infection, strategies that prevent PID are likely to be cost-effective within a single year.