To evaluate the long-term effects of dopamine agonists in the treatment of macroprolactinoma, we studied prolactin levels and tumor size for 30 to 88 months (57 +/- 14, mean +/- S.D.) in 38 patients treated with bromocriptine or lisuride. Elevated prolactin levels became normal in 30 patients, and the tumor shrank in 29. After two years of treatment, we attempted to reduce the maintenance dose (5 to 20 mg of bromocriptine per day or 0.4 to 0.8 mg of lisuride per day); in 21 patients no changes in prolactin levels or tumor size were observed over 6 to 52 months with 0.625 to 10 mg of bromocriptine per day or 0.05 mg of lisuride per day. However, it was possible to withdraw the drug in only one patient. We conclude that dopamine agonists are usually effective treatments for macroprolactinoma and that after a response has been obtained, it can be maintained in many patients with a greatly reduced dose.