A noninvasive technique, color Doppler flow imaging, was used to study the predictive value for future episodes of thrombosis in vascular access (polytetrafluoroethylene [PTFE] grafts and radial-cephalic fistulae) in a series of chronic hemodialysis patients. Patients with grafts were classified into three groups based on maximum constriction of the vascular access diameter: narrowing of greater than 50% (group I), narrowing of 30% to 50% (group II), and no narrowing or narrowing less than 30% (group III). More than half of the patients in group I suffered vascular access thrombosis within 6 months, while only 9% to 13% of patients in groups II and III had episodes of thrombosis during the same time interval. Decreased access volume flow (AVF) also correlated with episodes of thrombosis in these patients. No correlation could be demonstrated between recirculation and future thrombosis. No patient with a fistula had narrowing greater than 30% and none had vascular access thrombosis during the 6-month follow-up.