The 121I content in the thyroids of staff members working with 125I at Malmo University Hospital has been measured regularly during a period of 20 y. The measurements were made with a specially designed Nal(TI) detector, 1.5 mm thick and 124 mm in diameter. Four categories of personnel have been studied: chemists performing routine labeling of mainly proteins and hormones with 125I and using high activities (tip to 74 MBq per single labeling); laboratory staff working with in vitro techniques (e.g., radioimmunoassay) using low activities of 125I (approximately 1 kBq in each sample); nuclear medicine staff working with intravenous injections of 125I-labeled radiopharmaceuticals (up to 4 MBq per injection) in patients; administrative staff working in the same department as the above categories, but not handling 125l. Staff from another department, not working with 125I, were used as a control group. Duting the period 1977–1990, the chemists had a mean 125I activity of 1.5 kBq in their thyroids, giving a yearly absorbed dose of 10 mGy to this organ. An individual value of 7.4 kBq (53 mGy y−1) was recorded at one occasion. The mean value of the activity for personnel working with in vitro techniques, mainly radioimmunoassay, was 140 Bq, with an individual value of 4 kBq recorded in this group. The nuclear medicine staff exhibited 30 Bq 121I in their thyroids, on average. Staff members not working with 125I, but working in the same department as someone in the categories above, had up to 25 Bq, indicating diffuse, low-level contamination of 125I in the department. Control subjects from the hospital, not working in any department handling 125I, showed no measurable amount of 125I in their thyroids (< 2 Bq). There is a trend towards decreasing internal contamination with time during the period 1977–1990 in spite of the constant use of 125I. This is supposed to be an effect of improved working conditions and active radiation protection information. The further decrease of internal contarnination from 1991 to 1996 is due to a decreased use of 125I.