The distribution of mitotic chiasmata from the lymphocytes of three patients with Bloom's syndrome was studied by Q-banding. An estimate was made of the average brightness of the human chromosomes and of the brightness and length of the Q-regions. A strong relationship was found between the average darkness of a chromosome and its chiasma density. The shorter and darker Q-regions contained more chiasmata, with the exception of the chromosome tips which had lower chiasma densities. A few "hot spots" had particularly high chiasma densities. Centric regions contained 17% of the chiasmata. The data are consistent with the assumptions that chiasmata are favored in darker regions and at borders between light and dark regions.