The problem of whether rapid-cycling (RC) bipolar disorder is more frequently associated than non-rapid-cycling (NRC) bipolar disorders with thyroid dysfunction was investigated in two groups of 11 women matched for age and therapy. Seven patients in each group were under chronic lithium therapy. Both RC and NRC patients, as compared to euthyroid controls, showed a reduction in mean total and free thyroid hormone concentrations, subnormal values of free thyroxine being found in four RC and three NRC patients. No patient had supranormal baseline thyroid stimulating hormone (TSH) values, but an exaggerated TSH response to thyrotropin releasing hormone was found in three RC and two NRC patients: all these patients had been receiving lithium therapy for more than one year. No differences in the prevalence of goiter and thyroid-directed autoantibodies were observed in the two groups. These data confirm that bipolar disorder, especially during treatment with lithium, is associated with at least subclinical hypothyroidism, and suggest that RC patients do not differ from NRC patients in the prevalence of spontaneous or lithium-induced thyroid hypofunction. Lithium-induced hypothyroidism is likely to be related to the length of treatment.