Chromosome analyses were carried out on lymphocytes from 11 persons with annual exposures to gamma rays below the occupational limit of 50 mSv, employed in a hospital department of radiotherapy and nuclear medicine. Cytogenetic damage indicated radiation exposure significantly above background in six cases. In four of them a dose estimate between 30 and 275 mSv with 95% confidence was derived, reflecting their accumulated occupational exposures. In one case a partial body exposure to about 30% of lymphocytes was inferred and inquiries revealed previously unknown radiotherapy. In order to avoid misinterpretation of cytogenetic data, reliable chromosome dosimetry requires that the individual's radiation history must be known.