PURPOSE. Osteochondral talar defects (OCDs) are sometimes located so far posteriorly that they may not be accessible by anterior arthroscopy, even with the ankle joint in full plantar flexion, because the talar dome is covered by the tibial plafond. It was hypothesized that computed tomography (CT) of the ankle in full plantar flexion could be useful for preoperative planning. The dual purpose of this study was, firstly, to test whether CT of the ankle joint in full plantar flexion is a reliable tool for the preoperative planning of anterior ankle arthroscopy for OCDs, and, secondly, to determine the area of the talar dome that can be reached by anterior ankle arthroscopy. METHODS. In this prospective study, CT-scans with sagittal reconstructions were made of 46 consecutive patients with their affected ankle in full plantar flexion. In the first 20, the distance between the anterior border of the OCD and the anterior tibial plafond was measured both on the scans and during anterior ankle arthroscopy as the gold standard. Intra- and interobserver reliability of CT as well as agreement between CT and arthroscopy were assessed by intraclass correlation coefficients (ICCs) and a Bland and Altman graph. Next, the anterior and posterior borders of the talar dome as well as the anterior tibial plafond were marked on all 46 scans. Using a specially written computer routine, the anterior proportion of the talar dome not covered by the tibial plafond was calculated, both lateral and medial, indicating the accessible area. RESULTS. The distance between the anterior border of the OCD and the anterior tibial plafond ranged from −3.1 to 9.1 mm on CT and from −3.0 to 8.5 on arthroscopy. The intra- and interobserver reliability of the measurements made on CT-scans were excellent (ICC > 0.99, p < 0.001). Likewise, agreement between CT and arthroscopy was excellent (ICC=0.97; p < 0.001); only one patient showed a difference of more than 2.0 mm. The anterior 47.3 ± 6.8% (95%CI, 45.2–49.3) of the lateral talar dome, and 47.7 ± 7.0% (95%CI, 45.7–49.8) of the medial talar dome was not covered by the tibial plafond. CONCLUSIONS. Computed tomography of the ankle joint in full plantar flexion is an accurate preoperative planning method to determine the arthroscopic approach for treatment of OCDs of the talus. Almost half of the talar dome is directly accessible by anterior ankle arthroscopy