Thyroid function, the clinical occurrence of goitre and ultrasonically determined thyroid gland volume were investigated in 23 patients with phenytoin- and 28 patients with carbamazepine-treated convulsive disorders and compared with matched healthy controls. In the phenytoin treated group median thyroid volume was 26 ml (range 14-57 ml) compared to 17 ml (range 8-41 ml) in the controls (P less than 0.01). Ten patients and four controls had a goitre (NS). Median serum T4 and FT4I levels were reduced, serum TSH level increased and serum T3, T3RU, FT3I and thyroglobulin levels unaltered compared with the controls. In the carbamazepine treated group median thyroid volume was 25 ml (range 13-66 ml) compared to 16 ml (range 9-44 ml) in the controls (P less than 0.01). Thirteen patients and three controls had a goitre (P less than 0.02). Median serum T4, FT4I and FT3I levels were reduced, serum thyroglobulin increased and serum T3, T3RU and TSH levels unaltered compared with the controls. The increase in thyroid size is probably a compensatory mechanism due to the low free thyroid hormones in serum caused by an increased hepatic degradation of thyroid hormones by phenytoin and carbamazepine.