G(0) human peripheral blood lymphocytes were X-irradiated to determine whether there is a direct relationship between radiation-induced dicentric chromosomes and the triggering of apoptosis. Immediately after X-ray exposure, control and irradiated lymphocytes were analyzed for viability, apoptosis and chromosome damage using the premature chromosome condensation technique. A batch of lymphocytes was kept in liquid holding for 48 h and then loaded on Ficoll-Paque medium to separate apoptotic (high-density) and normal (normal-density) cells. Then the same end points were analyzed in high-density and normal-density fractions of control and irradiated lymphocytes. After 48 h of liquid holding, the majority of apoptotic cells contained dicentric chromosomes. These results demonstrate that in human lymphocytes, the type of chromosome damage influences the induction of programmed cell death and provide direct evidence that cells bearing dicentrics are eliminated by apoptosis. G0 lymphocytes are the most common tissue used in biodosimetry studies, and the amount of chromosomal damage detected depends on the time between exposure and sampling. Since the radiation-induced apoptotic cells show the presence of dicentrics, radiation-induced damage can be underestimated. These results may have relevance in evaluations of the efficacy of radiotherapy based on the frequencies of chromosomal aberrations.