To define the role of air plethysmography (APG) in the clinical diagnosis and quantitative assessment of chronic venous insufficiency (CVI), APG studies were performed on 582 limbs in 291 patients with signs and symptoms of CVI. One hundred and thirty-one limbs were classified into group I (no evidence of CVI), 291 into group II (mild CVI), and 160 into group III (advanced CVI). On APG, the mean venous filling index (VFI) was 1.45 mL/s, 3.90 mL/s, and 5.25 mL/s in groups I, II, and III respectively (p<0.05). The mean ejection fraction (EF) and mean residual volume fraction (RVF) also showed significant differences between the limbs with CVI and the contralateral normal limbs, but the values were similar for the different severities of CVI limbs. The amount of overlap in VFI, EF, and RVF values among the clinical groups was considerable. Discrimination analysis derived a VFI value of 2.67 mL/s as a cutoff point between normal limbs and limbs with CVI, with a positive predictive value of 96%. In conclusion, APG is a simple and noninvasive test for quantitative assessment of the different components of CVI, valvular reflux, and calf muscle pump function. However, only VFI correlated significantly with the severity of CVI. VFI, with its high positive predictive value, may be useful in diagnosis of CVI, and it may serve as an objective quantitative measurement for monitoring the effect of treatment.