The evidence for the association between IUD use and PID is reviewed based on the results of case-control and cohort studies. Both types of epidemiologic studies are described briefly. Most case-control studies are hospital-based and show that IUD users have a higher risk of having PID compared to women using other contraceptive methods. The results of case-control and cohort studies, however, are not in agreement. Considered collectively, cohort studies show that the PID rates are the same for users of all types of IUDs. This rate is similar to the PID rate in the general population not associated with abortion, childbirth or surgery.