To evaluate whether abnormal production of type III collagen, the characteristic biochemical feature of patients with type IV Ehlers-Danlos syndrome, consistently predisposes to mitral valve prolapse, the authors evaluated the family of a proband with classic type IV Ehlers-Danlos syndrome. Production of type III collagen was assessed with the use of cultured skin fibroblasts. Mitral valve prolapse was detected by M-mode and two-dimensional echocardiography. Biochemical abnormalities in the production of type III collagen and echocardiographic findings of mitral valve prolapse were completely concordant. All patients with abnormal production of type III collagen had mitral valve prolapse and all subjects with normal production of type III collagen had entirely normal echocardiograms. Six of the eight patients with abnormal production of type III collagen had subtle cutaneous abnormalities. The consistent association of abnormal production of type III collagen and mitral valve prolapse in this family suggests that this abnormality of collagen may give rise to mitral valve prolapse.