Fifty-three psychiatric patients who had been receiving treatment with lithium continuously for more than two years were examined to estimate the prevalence of lithium-induced hypothyroidism. It was found to be 20 per cent among women. No men were affected among these patients. In order to study the characteristics of the disorder further cases were drawn from another population. One third of the patients developed hypothyroidism during their first year of treatment, others not until after 9 years. About two thirds of the female patients with hypothyroidism had thyroid antibodies. All cases with lithium-induced hypothyroidism showed elevated levels of serum thyrotropin, which in our experience is the laboratory examination of choice in these as well as other cases of ‘primary’ hypothyroidism. Since the probability of detecting these cases at a given control visit was found to be low, we feel that such visits need not include extensive laboratory investigations. Hypothyroid patients responding well to lithium treatment should continue their medication combined with appropriate thyroxine substitution.