In this article, we propose a vision-based kinematic calibration algorithm for Stewart-Gough parallel structures. Information on the position and orientation of the mechanism legs is extracted from the observation of these kinematic elements with a standard camera. No workspace limitation, nor installation of additional proprioceptive sensors are required. The algorithm is composed of two steps: the first one enables us to calibrate the position of the joint centers linked to the base and possibly evaluate the presence of joint clearances. The kinematic parameters associated to the moving elements of the platform are calibrated in a second step. The algorithm is first detailed, then an experimental evaluation of the measurement noise is performed, before giving simulation results. The algorithm performance is then discussed.