The objective of the present work was to know the guidelines of antibiotic prophylaxis in bacterial endocarditis used in different spanish health centers. A general dental practitioner asked orally in 50 Cardiology and/or Internal Medicine departments throughout Spain which prophylaxis should be administered to a patient with a mitral valve prosthesis before a dental extraction. The results obtained showed that only 36 (72%) departments used the latest prophylactic guidelines recommended by the American Heart Association or the British Society for Antimicrobial Chemotherapy. Among penicillin allergic patients the antibiotic of choice was erythromycin (60%) followed by clindamycin (28%), although administered at very different dosages (11 and 3, respectively). Only in 44% of the surveyed departments did the guidelines for allergic and non allergic patients correspond to the recommended protocol by the same study group. The controversy generated regarding the prophylactic indications for bacterial endocarditis might partially account for the results obtained in this study.