We validated automated oscillometric measurement of the ankle-to-arm ratio of systolic blood pressure (ankle-brachial blood pressure index, ABI) against the manual approach, which requires trained observers and has an intra-observer variability of approximately 10%. A single observer measured ABI in 105 subjects (age range, 20-80 years; women, 51.9%). The manual approach involved the auscultatory measurement of systolic pressure at the brachial artery and Doppler measurements of systolic pressure at the anterior and posterior tibial arteries. The automated oscillometric VP-2000 (Omron Healthcare, Kyoto, Japan) was used to measure systolic pressure at the arm and ankle simultaneously. We applied Bland and Altman's methods to study reproducibility. Ankle-brachial blood pressure index averaged (+/-s.d.) 1.13+/-0.07 units and 1.12+/-0.10 units on automated and manual measurements, respectively. The absolute and percentage differences (P=0.64) between the two ABI measurements were 0.005 units (95% confidence interval, -0.015 to 0.024) and 0.60 (-0.69 to 1.04), respectively. The intra-observer variability of repeat ABI measurements was smaller on automated than on manual measurement (0.17 versus -0.84%; P=0.04). Automated ABI measurement by an oscillometric technique requires little observer training, and significantly increases the reproducibility of ABI estimates compared with the manual approach.