Up to 70% of cases of pelvic inflammatory disease do not have a known cause. We recruited 115 women who had presented to a clinic for sexually transmitted diseases in Nairobi, Kenya with pelvic pain that had persisted for 14 days or less, to look for an association between Mycoplasma genitalium and endometritis. With PCR, we detected M genitalium in the cervix, endometrium, or both in nine (16%) of 58 women with histologically confirmed endometritis and in one (2%) of 57 women without endometritis (p=0.02). Our results suggest that infection with M genitalium is strongly associated with acute endometritis in this population.