Polymicrobial endocarditis was very uncommon until ten years ago. However, since that time, at least 21 cases were reported, and 10 patients with this mixed infection were seen at our hospital. All, except one of these infections, occurred in patients who had undergone heart surgery or abused intravenous drugs. Although, generally clinically indistinguishable from mono-microbial endocarditis, these mixed infections carried a very high mortality rate (greater than 30 per cent), and an unusually large number of the patients (greater than 50 per cent) needed heart surgery either to control the infection or to repair cardiac defects resulting from the infection. The prognosis depended on the species rather than the number of organisms isolated and on aggressive antimicrobial and surgical therapy.