OBJECTIVE: To study the microbial characteristics of patients with pelvic inflammatory disease (PID) and the possible impact of an intrauterine device (IUD) on the microbial environment in women presenting with PID. METHODS: Case-control study, investigating 51 women with acute PID and 50 healthy women. Endocervical specimens for microbiological investigation were obtained at gynaecological examination. RESULTS: IUD users with PID had significantly more Fusobacteria spp. and Peptostreptococcus spp. than non-IUD users with PID. The finding of combinations of several anaerobic or aerobic microbes was associated with a significantly increased risk of PID and with complicated PID. In IUD users, the combinations of several anaerobic/aerobic microbes were associated with an increased risk of PID, irrespective of duration of IUD use. Long-term IUD use appeared to be associated with an increased risk of a PID being complicated. CONCLUSION: The finding of several anaerobic or aerobic microbes appears to be associated with PID in users of IUD.