Fugitive acromegaly is most commonly caused by pituitary acidophil stem cell adenomas, and is characterized by a relatively short clinical history, a large and locally invasive tumor, and relatively low hormonal activity. Here, we report an unusual case of fugitive acromegaly that initially presented as invasive prolactinoma. A 48-year-old man with a huge pituitary mass extending to the suprasellar area was referred to our hospital in December 2007. He had undergone transsphenoidal surgery in November 1999 because of a large invasive prolactinoma. The tumor had grown progressively, despite therapy with dopamine agonists. Subtle features of acromegaly were noted and serum IGF-1 levels were high (733 ng/ml). An oral glucose tolerance test revealed that basal and nadir levels of growth hormone (GH) were 1.56 and 1 ng/ml, respectively. As a therapeutic trial, long-acting octreotide (20 mg IM, monthly) was added, and the tumor size markedly reduced within 6 months on magnetic resonance imaging examination. Immunohistochemical staining of the tumor tissue obtained at the surgery in 1999 showed positive staining for GH and prolactin (PRL). Double immunofluorescence staining showed a mixed positivity for GH and PRL in the majority of tumor cells; however, the two hormones colocalized in a minority of tumor cells, indicating that the tumor was composed of three different cell types (GH, PRL, and GH/PRL). The diagnosis of fugitive acromegaly was initially overlooked in this patient because of normal serum GH levels and a lack of acromegalic features, although histological evidence for GH production was present. IGF-1 determinations would be helpful for the diagnosis of fugitive acromegaly.