Fifteen patients under age 50 years with central retinal vein obstruction (CRVO) and no obvious predisposing ocular or systemic condition underwent a comprehensive medical investigation, including platelet function studies and, in 11 cases, echocardiography. Only 1 patient was found to have a systemic disease (hyperlipoproteinemia), but 11 patients (73%) were found to have abnormalities of platelet function, usually platelet coagulant hyperactivity. Of the 11 patients who underwent echocardiography, 7 (64%) were found to have mitral valve prolapse, a condition predisposing to vascular occlusion, and 4 (60%) of the 7 had abnormalities of platelet function. These findings suggest that mitral valve prolapse contributes to platelet hyperactivity and, ultimately, CRVO.