T cell clones in patients with ataxia telangiectasia (AT) and T cell prolymphocytic leukemia (T-PLL) have identical chromosome abnormalities, namely inv(14)(q11q32), t(14;14)(q11;q32) and t(X;14)(q27;q11). In T-PLL and AT developing T cell leukemia, the above abnormalities occur frequently together with trisomy for 8q. We postulated that the additional abnormalities of chromosome 8, where the c-myc oncogene is mapped to 8q24, may play a role in the development of overt leukemia. DNA analysis using the CD1A c-myc probe did not reveal rearrangements of the c-myc gene by Southern blotting. We have used a monoclonal antibody for the c-myc protein to investigate the level of expression in 11 patients with T-PLL and two with Sezary cell leukemia and compared it with levels seen in normal lymphocytes. Significantly higher levels were observed in patients compared with controls (P < 0.0001). The highest levels of c-myc were seen in eight cases with trisomy for 8q resulting from an i(8q). One patient was investigated before and after treatment. In the active state, c-myc showed a level of 64.36 units (range 20-200). After treatment a residual population of malignant cells showed a c-myc level of 155 (range 90-280). This study suggests that the increased expression of c-myc as a result of trisomy for 8q may have a role in the pathogenesis of de novo T-PLL and T cell leukemia supervening AT and that there may be a correlation between c-myc levels and resistance to therapy.