Thyroid function was evaluated in a group of 53 patients who had taken oral contraceptive pills for an average of 153 consecutive cycles. We found the same alterations reported previously by other authors in the PBI and T3. test. Values for131I in 40% of the patients were abnormally low, a finding that has not been reported previously. Levels of total thyroxine and free thyroxine (not index) were relatively unchanged. On the basis of this study, we can say that although individual tests show alterations, none of the patients was considered to have a thyroid disease. There is no evidence to indicate a definite relationship between prolonged use of oral contraceptive medication and the development of thyroid disease.