The present report is a review of data assuming an etiological relationship between pelvic inflammatory disease (PID) and Mycoplasma hominis. Thus the organism can be isolated from the vagina/cervix more frequently in PID patients than in any other clinical group, i.e., in half to three-fourths of all such cases. One-fourth of PID patients develop a significant antibody response to M. hominis during the course of the disease. The antibody response can be detected by indirect hemagglutination tests. Grivet monkeys infected experimentally with M. hominis develop PID, predominantly parametritis; the infection seems to spread via lymphatics to the parametria. These animals develop a significant antibody response. The animals, like naturally infected women, develop a marked increase in the serum level of IgM. In tissue cell cultures of human fallopian tubes experimentally infected with M. hominis, a decrease of the mucociliary wave activity occurs. So far, few clinical data support an etiological role for Ureaplasma urealyticum in PID. In grivet monkeys, the organism does not produce PID.