The diagnosis of acromegaly is suspected based on the typical clinical presentation and is subsequently confirmed biochemically by elevated GH and IGF-I concentrations. We report three female patients with pituitary tumors who presented without any signs or symptoms of acromegaly but with elevated IGF-I levels. Plasma GH was measured every 10 min for 24 h, and an oral glucose tolerance test was performed. All patients had abnormally elevated mean and trough plasma GH levels as well as post-glucose nadir GH concentrations. All patients had magnetic resonance imaging scans revealing pituitary tumors and underwent transsphenoidal surgery. Histologically, they had GH-producing pituitary tumors. Plasma IGF-I levels returned to normal in two patients after surgery. Some pituitary adenomas are true GH-secreting tumors despite not being accompanied by obvious clinical stigmata of acromegaly. Natural history of this disease is unknown because of the small number of reported patients and inconsistent results of biochemical testing. Based on the results of this and previous reports, we propose that all patients with known pituitary tumors, especially younger women with normal or mildly elevated prolactin level, be evaluated for GH excess.