Neisseria subflava is a rare cause of bacterial endocarditis. Only seven cases have been identified in the world literature if strict criteria as to organism classification and of endocarditis are applied to individual case reports. The first reported case in an intravenous drug user is described. In addition, findings on serial two-dimensional and M-mode echocardiography performed during and after the treatment period are presented. This patient's recovery with anti-microbial therapy alone despite evidence of aortic and significant mitral valve involvement is in accord with results of other antibiotic-treated cases.