[en] Chromosome dosimetry has become established in radiological protection as a useful technique to supplement physical methods in the event of a suspected or confirmed accidental overexposure to external ionising radiation. The main outstanding problems of the technique are discussed and the 540 cases which have been examined in the UK by the NRPB Laboratory from 1967-1980 are reviewed. The dose response data used to convert aberration yield to equivalent whole-body dose are described. Factors influencing the sensitivity and accuracy of the dose estimates are discussed. These include statistical uncertainties, variability in response between exposed individuals, lymphocyte life-spans, non-uniform irradiation and inter-laboratory variability. In considering future developments it is suggested that the investigation of a serious overexposure anywhere in the world could now be assisted by one of the several laboratories which have experience with this technique. (author)