This research explored the onset of stereopsis, the ability to perceive depth from the different views provided by the two eyes. In a longitudinal study, infants were tested weekly from 6 to 20 weeks of age. The primary goal of the study was to establish the onset and the early development of sensitivity to uncrossed horizontal disparity. The infant participants were shown dynamic random dot stereograms displaying two squares, one with uncrossed horizontal disparity (0.5°) and one with vertical disparity (0.5°). The stimuli were presented on an autostereoscopic monitor. We used two methods, the forced-choice preferential looking (FPL) method and the classical natural preference (CNP) method, to measure whether the infants preferred the uncrossed over the vertical disparity display. According to the FPL data, the mean relative preferences for horizontal over vertical disparity were significantly greater than chance probability (0.50) from 13 weeks of age onward. With the CNP method we found significant preferences for uncrossed horizontal disparity from 15 weeks onward. The FPL method was hence more sensitive than the CNP method as it indicated an earlier onset of responsiveness to stereoscopic information.