Biological dosimetry by measuring chromosomal aberrations in peripheral blood lymphocytes is a useful tool to estimate heath risks arising from radiation exposure. Biological dosimetry for exposure to densely ionizing radiation becomes increasingly important because of the increasing use of accelerated heavy ions for cancer therapy. When radiation sources are handled, the danger of an accidental (over-)exposure must be kept as small as possible; however, it can never be completely averted. Thus, the broader application of heavy ion radiation is associated with an increasing need for information about the biological dosimetry of densely ionizing radiation. The in vivo results from patients undergoing radiotherapy and the in vitro dose-effect curves that were obtained during this work provide a useful extension for the biological dosimetry of densely ionizing radiation.