We randomized 103 patients (68 arteriovenous [AV] fistulas, 35 polytetrafluoroethylene [PTFE] grafts; mean follow-up 197 days) to monthly measurement of access flow (QAT), monthly measurement of static venous pressure (VPS), or no monthly monitoring (control patients) to determine whether access thrombosis would decrease. Patients with access flow <750 cc/min or with static venous pressure ≥ 0.5 were referred for angiography and angioplasty of stenotic lesions ≥ 50%. Six of sixty-two (9.7%) of monthly monitored patients (MM) developed access thrombosis vs. 9 of 41 (22%) of control patients (p < 0.05). Fewer MM patients developed thrombosis in AV fistulas (2.4% [2 of 42] vs. 15.4% [4 of 26] control patients; p < 0.05). Monthly monitored patients had fewer thrombotic episodes than control patients (19 vs. 125 per 100 patient-years; p < 0.01). Thrombosis rates were lowest in patients receiving monthly access flow measurement (5.9 [QAT] vs. 30.3 per 100 patient-years [VPS]; p < 0.05). In conclusion, intervention based on monthly access flow measurement or static venous pressure decreased hemodialysis access thrombosis. Measurement of access flow tended to result in lower thrombosis rates than after static venous pressure. We believe that monthly access flow measurement will ensure the lowest incidence of thrombosis and decrease the cost of access maintenance. ASAIO Journal 1999; 45:147–150.