Results from three experiments using spatial forced-choice techniques show that an illusory contour improves the detectability of a spatially superimposed, thin subthreshold line of either contrast polarity. Furthermore, the subthreshold line is found to enhance the visibility of the illusory contour. Stimuli which do not induce illusory contours, but reduce uncertainty about the spatial position of the line, give rise to a slight detection facilitation, but the threshold of 75% correct responses is not attained. The data indicate that superimposing illusory contours and subthreshold lines produces interactions which are similar to classic subthreshold summation. They thus provide psychophysical evidence for the functional equivalence of illusory contours and real lines suggested by recent neurophysiological findings.