Objective To study the microbiology and long term prognosis of pelvic inflammatory disease (PID). Design A prospective study of women with laparoscopically confirmed PID. Setting Teaching hospital in central London. Subjects 23 women with PID. Outcome measures Microbiological investigations at the time of diagnosis and at follow up; subsequent fertility and the occurrence of pelvic pain. Results PID diagnosed by laparoscopy was regarded as moderate to severe in 15 cases. Neisseria gonorrhoeae, Chlamydia trachomatis, Mycoplasma hominis and Ureaplasma urealyticum or a combination of these micro‐organisms were detected most frequently in the cervix, less often in the endometrium and least in the tubes, C. trachomatis being the micro‐organism found most commonly in the tubes. There was serological evidence of acute chlamydial infection in 13 of 20 cases in which paired sera were available and a serological response to M. hominis in 9 of 12 cases in which it was possible to evaluate the results. On the basis of microbiological and serological results, C. trachomatis appeared to be the most important aetiological agent in 10 cases, N. gonorrhoeae in four cases, M. hominis in three cases and U. urealyticum in none; in five cases, two of these microrganisms appeared to be of equal importance. After 1 to 3 years, 33% of the women were having difficulty conceiving and 56% continued to complain of pelvic pain. Conclusion It is difficult to be certain of the infectious cause of PID in any given case. However, the evidence that N. gonorrhoeae and C. trachomatis are pathogens is very strong. M. hominis may be responsible for a few cases on its own or together with other micro‐organisms.