Silent somatotroph adenomas are defined as tumors showing morphological features consistent with GH production, but no clinical evidence of GH excess. We report here the case of a 46-yr-old woman with a large pituitary macroadenoma, slightly elevated serum GH levels, high serum insulin-like growth factor-I levels, and abnormal GH dynamics, but no acromegaly. The endocrinological abnormalities receded after transphenoidal surgery despite tumor persistence, as shown by neuroimaging. The reverse hemolytic plaque assay, performed for the first time in a silent GH cell adenoma, demonstrated that the number of GH cells releasing GH and the amount of GH discharged from individual cells were less than those in clinically functioning somatotroph adenomas. Thus, it is conceivable that this tumor secreted only small quantities of GH and for only short periods, providing an explanation for the lack of acromegaly. It appears that silent somatotroph adenomas do not represent a distinct entity. It is more likely that there is a continuous spectrum from clinically functioning, sparsely granulated somatotroph adenomas with high serum GH levels to silent somatotroph adenomas with normal serum GH levels. The cause of the lack of GH oversecretion in silent GH cell adenomas has yet to be elucidated.