An important potential source of error in the echocardiographic diagnosis of mitral valve prolapse has been identified, namely a systolic hammock like pattern of the anterior and/or posterior mitral leaflet echoes, similar to that associated with true mitral valve prolapse, produced artifactually when the ultrasonic transducer is angulated inferiorly. Utilizing a modified, more specific technique the authors characterized the mode of inheritance and familial prevalence of this disorder. Among 74 subjects, composed of 57 first degree relatives and 17 propositi with mitral valve prolapse, mitral valve prolapse was detected in 27 of 57 (47%) of the first degree relatives. 53% Of female and 36% of male progeny of propositi were affected. Furthermore, familial transmission occurred from propositi to both sexes. Results of this study indicate that mitral valve prolapse is transmitted in an autosomal dominant mode with reduced male expressivity and a familial prevalence of 47% and that appropriate echocardiographic techniques must be employed to avoid a high incidence of false positive diagnosis.