When a patient with mitral valve prolapse was found to have myotonic dystrophy, his family was studied to ascertain the occurrence of these abnormalities. Of 25 relatives screened, 8 had evidence of both myotonic dystrophy and mitral valve prolapse and 2 had evidence of myotonic dystrophy alone. Thirteen patients had evidence of neither condition and 1 had mitral valve prolapse alone. The association of these 2 conditions within a family was not previously observed and valvular heart disease was not recognized in myotonic dystrophy. This association should be looked for in other patients.