Aim: The in vivo comparison of the accuracy of two 3-dimensional quantitative coronary angiography (QCA) systems. Methods: Precision-drilled plexiglass phantoms with five different luminal diameters (0.5-1.9 mm) were percutaneously inserted into the coronary arteries of four Yorkshire pigs. Twenty-one angiographic images of these stenotic phantoms were acquired for in vivo validation testing. Quantitative assessments of the minimum, maximum, and mean luminal diameters together with the minimum luminal area were determined using two 3D QCA systems, the CardiOp-B 1 and CAAS 5. Results: The CardiOp-B system significantly underestimated the minimum luminal diameter MLD whilst both systems significantly overestimated the maximum luminal diameter at the minimal luminal area (MLA) over the phantom's true value. The CAAS 5 system had a greater degree of accuracy/mm (mean difference 5 0.01 vs. 0.03) and precision/mm (SD 5 0.09 vs. 0.23) than the CardiOp-B in assessing the minimal LD. An increased precision/mm (SD 5 0.01 vs. 0.29) and accuracy/mm (mean difference 5 0.03 vs. 0.11) in the mean LD was observed with the CAAS 5. In comparing the MLA/mm 2 the CAAS 5 was more precise/mm 2 (SD 5 0.14 vs. 0.55) and accurate/mm 2 (mean difference 5 0.12 vs. 0.02) to the true phantom MLA compared to the CardiOp-B system. Conclusions: In a 21 phantom study, the CAAS 5 3D QCA system had a greater degree of accuracy and precision in both the luminal and area measurements than the CardiOp-B 3D QCA system. '