Five female patients with primary hypothyroidism and radiological evidence of a pituitary enlargement were studied before and after a mean of 30 months (range 12-83 months) treatment with thyroxine (T4). Before treatment, serum thyroid-stimulating hormone (TSH) levels were elevated in every patient (mean 392 mU/l, range 240-475) and prolactin levels in 4 (mean 79 micrograms/l, range 48-143 micrograms/l). CT scanning confirmed the presence of pituitary enlargement in the 4 patients studied, which was suprasellar in 3. The remaining patient had an enlarged fossa on a lateral skull radiograph. During treatment with T4, TSH and prolactin levels were normal in all. Complete disappearance of the enlargement was seen on follow-up scans in all patients and 1 developed an empty sella. The induction of a pituitary enlargement by primary hypothyroidism results from reversible hyperplasia of both the TSH and prolactin-secreting cells in most instances. Occasionally, however, hyperplasia of the thyrotrophs can occur in isolation and an empty sella can occur after successful treatment with T4. Thyroid function tests should be obtained in all hyperprolactinemic patients.