The distribution of the break points of sister chromatid exchanges (SCE) was compared with that of chromosome aberrations in Bloom’s syndrome by using differential sister chromatid staining and banding techniques. A comparison was made of the distribution in chromosomes 1, 2, and 3, since the exact identification of other chromosomes is difficult with the differential sister-chromatid staining technique. It was shown that SCE and chromosome breaks do not necessarily correlate as to location. Some chromosome break points, e.g., 1q21, lp36, 2q31, 3ql2, and 3pl3, were common with those of SCE, whereas others (at 1pl3, 2pll, 2qll, and 3qll) showed little or no SCE. SCE breaks were not observed in the centromeric regions. In addition, the SCE frequency was examined in Bloom’s syndrome cells with and without chromosome aberrations, and no significant differences of SCE frequency were observed between cells with chromatid- or chromosome-type of aberrations and those with normal complements. Banding analyses indicated a nonrandom distribution of chromosome breaks in the lymphocytes and marrow cells of the Bloom’s syndrome pa