Background Only a few cases of eustachian valve endocarditis have been reported. Whether the eustachian valve is an uncommon site for a vegetation to be attached or whether the disease is missed because a systematic approach to this valve is not routinely performed in the search for vegetations is not known. Methods Every patient suspected of having endocarditis undergoes a specific approach, which includes a systematic study of the eustachian valve. In 10 patients with large valves but without signs and symptoms of endocarditis, we identified 2 specific findings: width <3 mm and a regular oscillating movement. A blinded evaluation in the 10 control subjects and 30 patients with right-sided endocarditis, including the 5 with eustachian valve endocarditis, showed an agreement of 97% (39/40). Results Five of 152 patients with right-sided endocarditis were found to have eustachian valve vegetations (3.3%). Patients were young (age range 22-34 years) and all had predisposing factors (3 intravenous drug abusers, 2 central venous lines), fever, and septic pulmonary embolism. Staphylococcus aureus was cultured in all cases. Tricuspid involvement was found in 4 patients, and only 1 patient had isolated eustachian valve endocarditis. All patients did well with culture-guided antibiotics. Conclusions Our results suggest that eustachian valve endocarditis may be more frequent than is believed. Thus a systematic interrogation of the eustachian valve should be included in the echocardiographic examination of a patient suspected of having endocarditis. (Am Heart J 2001;142:1037-40.)