We determined serum T4 and serum TSH serially in a cohort of patients given lithium treatment for up to 6 years; the total lithium exposure time was 409 years and the average serum lithium concentration 0.69 mmol/l. T4 showed a small and not significant fall at 6 months and returned to the pre-lithium level at 12 months. Hereafter, T4 rose gradually and after 6 years of lithium treatment T4 was 53% higher than the pre-lithium value. TSH was significantly increased at 6 and 12 months and then returned to the pre-lithium level. Eight patients required thyroxine treatment for lithium-induced hypothyroidism, i.e. 2 per 100 years of lithium exposure time. Single deviant values of T4 and TSH could be seen, followed by normal values. We suggest that TSH is determined at intervals during lithium treatment. It may be prudent to subject lithium-treated patients with abnormal thyroid values to re-examination and to abstain from starting thyroxine treatment on the basis of a single deviant value.