In this study we investigated the accuracy of monoplane and biplane quantitative coronary angiography in estimating the luminal dimensions, using intracoronary ultrasound as gold standard. Biplane angiography and intracoronary ultrasound were performed in 24 arterial segments. The end-diastolic intracoronary ultrasound frames were manually selected and segmented. In 2 end-diastolic X ray projections, quantitative coronary angiography was performed and a novel methodology was applied to register the segmented frames onto the processed angiographic images. The luminal areas determined by quantitative coronary angiography in 1 (monoplane) and 2 projections (mean) were compared with those determined by intracoronary ultrasound. The obtained correlation coefficients for the monoplane and mean estimations were 0.69 +/- 0.12 and 0.77 +/- 0.08, respectively. It would appear that by increasing the angle between the biplane projections, the correlation between intracoronary ultrasound and mean estimations improves. Our results provide evidence that orthogonal biplane angiography is more reliable and should be preferred to assess luminal dimensions.