Bromocriptine, a potent D2-dopamine agonist, suppresses growth hormone (GH) secretion in most patients with acromegaly and has been approved for the treatment of acromegaly. Here we report a patient with acromegaly who showed increased GH secretion after administration of bromocriptine. A 70-year-old man with acromegalic manifestation was admitted to our hospital because of a pituitary tumor invading to the right cavernous sinus detected by brain magnetic resonance imaging. Serum GH and insulin-like growth factor-I (IGF-I) levels were elevated in several occasions (GH: 15.0-51.7 ng/mL, reference range: <2.47 ng/mL; and IGF-I: 776-856 ng/mL, reference range: 57-175 ng/mL). Effect of bromocriptine on serum GH levels was then studied because pre-operative treatment with a D2-dopamine agonist was planned in order to reduce the tumor size and serum GH levels before surgery. After oral administration of 2.5 mg of bromocriptine, serum GH levels were unexpectedly increased from 30.7 ng/mL to 189 ng/mL, despite the fact that the levels of prolactin (PRL) were decreased from 4.2 ng/mL to 0.6 ng/mL. By contrast, serum GH levels were decreased by a somatostatin analogue, octreotide. Transsphenoidal surgery of the pituitary tumor was performed after treatment of octreotide. Histological analysis and immunohistochemistry revealed a GH-producing pituitary adenoma positive for D2-dopamine receptor. This case of acromegaly suggests that the preliminary test with a single administration of a short-acting D2-dopamine agonist, bromocriptine, is mandatory before the long-term therapy with a D2-dopamine agonist in patients with GH-secreting pituitary tumors.