Pituitary adenoma, removed surgically from a 22 old young man with normal serum growth hormone level, and no evidence of acromegaly by histology, immunohistochemistry and electron microscopy was studied. Ultrastructurally the tumor cells were shown to correspond to sparsely granulated somatotrophs and immunohistochemistry showed no, moderate, or little growth hormone. Clinically silent somatotroph adenomas so far are unresolved entities; although electron microscopy shows that they are consisted of sparsely granulated somatotrophs they do not secrete substantial amounts to raise growth hormone and IGF-1 blood levels. Further work is required to clarify the mechanisms accounting for the clinical and biochemical silence of these tumors. doi:10.4021/jmc416w