The vaginal discharge of women with bacterial vaginosis often has a prominent fishy odor. Intensification of this fishy odor by the addition of strong base to the vaginal discharge suggests that it could be due to trimethylamine, the substance responsible for the characteristic odor of spoiling fish. Samples were collected from 11 women with a vaginal discharge having a fishy odor and from 10 women with no detectable odor. Gas chromatographic analysis of headspace samples of alkalinized vaginal discharges indicated the presence of trimethylamine in all 11 samples with the fishy odor but not in the other samples. The chemical identity of trimethylamine was confirmed by gas chromatography-mass spectrometry of headspace samples from two vaginal discharge samples. It is concluded that trimethylamine is the primary cause of the fishy odor associated with bacterial vaginosis.