A typical extragalactic radio source is a simple double or elongated structure with a well-defined orientation on the sky. In 1972, Willson1 investigated the relative orientation of 74 extragalactic 3C radio sources mapped at the Cambridge 1-mile telescope, and noted that extended extragalactic radio sources tend to be parallel if they are separated by less than 10° on the sky. Since 1972, many more radio sources have been mapped with resolutions 10–20 times greater than that of the observations available to Willson. I have therefore reinvestigated this problem in two separate and largely independent samples of some 300 sources each, and report here that the alignment is present to a high significance level, provided that only the more distant sources are considered. The effect could be due to gravitational imaging if most of the matter in a closed universe is in thin filaments.