Intrauterine contraceptive devices (IUCDs) are thought to cause pelvic inflammatory disease by allowing vaginal bacteria to pass into the uterus along the tail of the device. In this study the uterine cavities of 22 women using an IUCD were examined by a multiple biopsy technique. All five uteruses with a tailless IUCD were sterile but 15 out of 17 with a tailed device contained bacteria. The bacteria had not reached the fundus and most were commensals. The bacteria were not introduced by insertion of the IUCD as bacteria were present in several cases long after insertion. No differences in bacterial count were found between monofilamentous and multifilamentous devices. Bacteria were cultured from only four devices, which suggested that the bacteria adhere to the endometrium and not to the device. The bacteria in the cavity represent interference by the tail with the protective mechanisms of the uterus, which explains the increase in pelvic inflammatory disease in IUCD users.