To what extent the unavailability of coronary artery bypass graft (CABG) anatomy complicates the performance of diagnostic coronary angiography has not been studied. The medical and catheterization records and coronary angiograms of 367 consecutive CABG surgery patients who underwent 394 diagnostic coronary angiographic studies from October 1, 2004, to May 31, 2007, were retrospectively reviewed. The patients' mean age was 65 ± 9 years, and 97% were men. The mean interval from CABG surgery to angiography was 8.3 ± 6.1 years. Patent left internal mammary artery grafts were found in 75%, and the mean number of patent grafts was 2.1 ± 1.0. Compared with angiograms with known CABG anatomy, angiograms with unknown CABG anatomy (26%) required significantly higher amounts of contrast (189 ± 7 vs 158 ± 4 ml), longer fluoroscopy times (14.0 ± 0.7 vs 10.6 ± 0.4 minutes), and more diagnostic catheters (3.0 ± 0.1 vs 2.5 ± 0.05) (p <0.001 for all comparisons). The unavailability of CABG anatomy remained associated with increased contrast, fluoroscopy, and catheter use after multivariate adjustment. Proximal anastomotic bypass markers were associated with lower contrast use but were seen in only 9% of patients. In conclusion, the unavailability of CABG anatomy significantly and independently increases the use of contrast, fluoroscopy, and catheters during diagnostic coronary angiography. Every effort should be made to obtain CABG anatomy before diagnostic angiography is performed.