The R-banding of more than 1100 lymphocytes and fibroblasts from 11 patients with ataxia telangiectasia (AT) showed rearrangements of chromosomes 7 and/or 14 in about 7% of the cells. Among these rearrangements, pericentric inversion of chromosome 7 was the most frequent, and the paracentric inversion of chromosome 14 not very rare. These inversions are believed to be fairly specific of AT, and their absence in the previously reported literature may be due to technical problems. Inversions were also observed in the lymphocytes of presumed heterozygote carriers with a lower frequency, and may be of some help for the detection of healthy heterozygote carriers of the AT gene.