The ankle/brachial blood pressure index (A/B PI) is important in the vascular laboratory assessment of peripheral vascular disease. However it is falsely elevated in diabetes, hence underestimating the true severity of disease. We have therefore examined the influence of diabetes on the A/B PI in 2092 patients, 538 with diabetes, all referred for evaluation of peripheral vascular disease. The prevalence of a raised A/B PI (greater than or equal to 1.5) in insulin-treated patients (18.3%) was much higher (p less than 0.001) than that in both non-insulin-treated diabetic patients (4.5%) and patients with no diabetes (2.8%). Insulin-treated patients with a duration of diabetes of greater than 30 years had a higher prevalence of raised A/B PI than those with a duration of less than or equal to 9 years. No significant age effects were seen.