The colonization of the female genital tract with Actinomyces is closely related to the presence of an IUD in the uterine cavity. This was confirmed in the present study, which shows an Actinomyces frequency of over 3% in users of IUDs, while non-users were free from the opportunistic invader. This frequency was higher with certain types of devices than with others. None of the patients with positive results had pelvic symptoms, but cases of serious infections have been reported in the literature. The length of IUD usage appears to be directly related to the incidence of this colonization. The possible pathogenesis of pelvic actinomycosis is presented; the clinical significance of asymptomatic colonization is discussed.