Soon after World War II, the problem of genetic hazards to populations from exposure to ionizing radiation became very apparent and substantial research efforts were launched. Chromosomal aberration production is one of the more pronounced effects of radiation at the cellular level. A great deal of information has been accumulated over the years. At first this was mainly from studies on plants and invertebrates. More recently, however, the development of various new techniques, mostly employing peripheral lymphocyte cultures, has led to an increasing amount of work on humans. Subsequent in vitro studies from different laboratories have shown that the aberration yields in human peripheral lymphocytes stimulated by PHA can serve as a quantitative biological indicator of radiation exposure. However, in vivo studies on healthy humans were limited to either the survivors of the Hiroshima and Nagasaki explosions or accidental exposures. More suitable sources for in vivo studies are therapeutically irradiated patients. Until now most of this work has been on patients with ankylosing spondylitis who received partial body radiotherapy. However, in the last few years very few patients have undergone radiotherapy for this disease. In this work therapeutically irradiated patients with Hodgkin's disease were investigated as an alternative model for in vivo studies. The long term cytogenetic effects of radiation using symmetric aberrations could not be carried out due to the paucity of such aberrations. This may have been due to the relative lethality of the large doses of X-rays absorbed by the lymphocyte stem cells during the treatment of Hodgkin's disease. This contrasts with the lower doses administered to patients with ankylosing spondylitis where symmetric aberrations have been observed many years after radiotherapy. It was also indicated that the use of asymmetric aberrations as a biological dosimeter may not be an entirely practical proposition either, at least in the near future. This is mostly because of the apparently differing rates of removal from the body of lymphocytes which have absorbed different amounts of radiation. This means that a mere knowledge of time since irradiation is not sufficient to predict the initial yield of chromosomal damage and hence the whole body equivalent dose absorbed. This suggests that the proposal of using chromosome aberrations induced by ionizing radiation as a biological dosimetry is still open for much further work. Other observations suggested that ionizing radiation is associated with in vivo reductions of sister chromatid exchanges and satellite associations.