A patient with a macroprolactinoma was treated with bromocriptine 15 mg daily. Both the size of the tumour as shown by computed tomography and the serum prolactin concentration decreased over several months but then increased. The dose of bromocriptine was increased to 40 mg daily but tumour growth continued, and the tumour was resected. Production of prolactin by cultured cells was not inhibited by high concentrations of bromocriptine, suggesting that regrowth of the tumour was due to cells resistant to dopamine agonist action. This case of regrowth of a prolactinoma during bromocriptine treatment after an initial reduction in size indicates the need for close surveillance especially of patients whose serum prolactin concentration fails to fall into the normal range with bromocriptine treatment.