A case of total A-V block observed in a 16 year old who had been suffering for two years from episodes of MAS and in whom further investigation revealed a prolapse of the rear flap of the mitral valve is reported. In the light of literature data, possible relations between the two conditions, A-V block and mitral valve prolapse are reviewed. In view of the youth of the patient, it is considered that the pathogenic agent underlying mitral valve prolapse (probable rheumatic aetiology) was also responsible for the conduction disturbance.