Visual perception critically depends on orientation-specific signals that arise early in visual processing. Humans show greater behavioral sensitivity to gratings with horizontal or vertical (0 degrees /90 degrees; 'cardinal') orientations than to other, 'oblique' orientations. Here we used functional magnetic resonance imaging (fMRI) to measure an asymmetry in the responses of human primary visual cortex (V1) to oriented stimuli. We found that neural responses in V1 were larger for cardinal stimuli than for oblique (45 degrees /135 degrees ) stimuli. Thus the fMRI pattern in V1 closely resembled subjects' behavioral judgments; responses in V1 were greater for those orientations that yielded better perceptual performance.