31 women with acute pelvic inflammatory disease, in whom serum antibodies to Chlamydia trachomatis and Mycoplasma hominis could not be detected, were examined for antibodies to Mycoplasma genitalium by a microimmunofluorescence technique. About 40% of them had a fourfold or greater change in the titre of antibody during the one-month period after the onset of disease. This evidence implicates M genitalium in the aetiology of pelvic inflammatory disease.