To assess the clinical features which might influence therapy, we studied 43 patients with late prosthetic valve endocarditis (LPVE). Twenty patients (47 per cent) survived. Of patients with streptococcal LPVE 61 per cent (11 of 18) survived compared to 36 per cent (nine of 25) of the patients with nonstreptococcal LPVE (p less than 0.10). Among patients with new regurgitant murmurs 33 per cent (nine of 27) survived versus 69 per cent (11 of 16) with such murmurs (p less than 0.03). Of patients with moderate to severe congestive heart failure (CHF) 16 per cent (three of 19) survived compared to 71 per cent (17 of 24) with mild or no CHF (p less than 0.001). The concurrence of two of these three features, i.e., nonstreptococcal etiology, a new regurgitant murmur or moderate to severe CHF, was associated with a mortality rate of 50 to 90 per cent. Persistent fever during therapy, a regurgitant murmur, atrioventricular conduction disturbances and relapse frequently reflected myocardial invasion. In view of the poor outcome with medical therapy and late reoperation, early surgical intervention should be considered when two of the three features noted are present or when myocardial invasion is suspected.