High resolution studies of structural rearrangements were carried out using the G-band technique. A total of 220 breakage points were identified within individual bands from 117 unrelated cases born with a structural rearrangement. Breakage points were not evenly distributed along chromosomes in terms of G-band patterns. There was an excess involvement of light bands and a striking lack of dark bands in both reciprocal translocations and inversions. In reciprocal translocations, the middle part of a chromosome arm has less chance of being the site of an exchange than the terminal and centromeric parts. The implications of these results are briefly discussed.