A case of endocarditis caused by Actinobacillus actinomycetemcomitans is described and 18 cases are reviewed. Clinical features of endocarditis caused by A actinomycetemcomitans and antibiotic susceptibility are reviewed. Because of the relative resistance of A actinomycetemcomitans to penicillin and ampicillin, high doses of either penicillin or ampicillin should be combined with streptomycin or gentamicin for four to six weeks.