In a patient affected by ataxia telangiectasia (AT), an invading clone with a t(14;14) was found in PHA-, but not in pokeweed-stimulated, lymphocytes. With high resolution R-banding, the proximal breakage was visualized at the junction of bands q11.1-q11.2. This breakpoint differs from that (q12) of noninvading rearrangements of chromosome 14 in AT and non-AT patients. These differences are discussed.