In this paper, we evaluate the accuracy and robustness of intensity-based 2D-3D registration for six image similarity measures using clinical gold-standard spine image data from four patients. The gold-standard transformations are obtained using four bone-implanted fiducial markers. The three best similarity measures are mutual information, cross correlation, and gradient correlation. The mean target registration errors for these three measures range from 1.3 to 1.5 mm. We believe this is the first reported evaluation using clinical gold-standard data.