The early diagnosis of acromegaly may be difficult when serum levels of growth hormone are minimally elevated and imaging of the pituitary gland fails to show an adenoma. However, transsphenoidal surgery has the greatest chance of cure at this stage. We therefore investigated the value of sampling petrosal sinuses for measurement of growth hormone in this group of patients. Simultaneous bilateral sampling of the inferior petrosal sinuses to measure serum concentrations of growth hormone was performed in five patients suspected of having acromegaly but with nondiagnostic CT scans (n = 5) and MR images (n = 3) of the pituitary gland. Levels of growth hormone from the petrosal sinuses were five to 36 times greater than levels in the peripheral veins in all five patients, and three of four showed a marked response to growth hormone-releasing hormone. During transsphenoidal surgery, growth hormone-producing microadenomas were resected completely in four patients. In the fifth patient, a left-sided microadenoma had invaded the cavernous sinus and could not be resected completely. Lateralization of the adenomas within the pituitary gland on the basis of differences in levels of growth hormone between the two petrosal sinuses was not completely reliable. Elevated levels of growth hormone in selective samples from the inferior petrosal sinuses can help support an early diagnosis of acromegaly when peripheral growth hormone levels and imaging are not diagnostic.