The initial prescriptions and later adjustments of thyroxine (T4) replacement have been analysed in 2246 hypothyroid patients who have been monitored by a thyroid follow-up register for a mean period of 6 years (range 1-13 years). In 465 (21%) patients of 65 years or over, initial T4 doses were on average only 20 micrograms lower than in younger patients. However, only 40% of the elderly were taking 200 micrograms of T4/day or more, compared with 60% of the younger patients. Over 10 years, the predicted proportion of patients who have a downward adjustment of their T4 prescription is 3%, but there is no difference in recognized overtreatment between older and younger patients. The findings suggest that the majority of the elderly may receive larger doses of T4 than are required for optimal replacement. There are strong reasons for the standardization of thyroxine prescribing in the elderly and for the provision of routine surveillance of thyroxine replacement through the type of follow-up system used in this study.