Somatotroph adenomas of the pituitary secrete growth hormone in excess and are associated with acromegaly. Morphologically, they can be separated into two entities, densely and sparsely granulated variants. It has been shown that a number of somatotroph adenomas produce α-subunit of glycoprotein hormones; however, it is not clear whether α-subunit production correlates with tumor cell morphology. We studied 32 surgically removed pituitary somatotroph adenomas in tissue culture to determine structure-function correlations of growth hormone and α-subunit production. All tumors were classified on the basis of detailed histological, immunocytochemical and electron-microscopic studies. Fifteen tumors were densely granulated and 1 7 were sparsely granulated. In addition to growth hormone, all 15 densely granulated tumors released α-subunit in vitro, whereas of the 1 7 sparsely granulated tumors only 4 released α-subunit moreover, the mean baseline levels of α-subunit were significantly higher in densely granulated adenomas than in sparsely granulated adenomas. Parallel response of release of both hormones was found during stimulation with growth hormone-releasing hormone or thyrotropin-releasing hormone and during suppression with somatostatin or bromocriptine in densely granulated tumors. α-subunit response to stimulation or suppression could not be determined with significance in sparsely granulated tumors because of low basal levels. The results indicate that α-subunit production and release is characteristic of densely granulated somatotroph adenomas and that α-subunit is coregulated with growth hormone by adenohypophysiotropic substances; in contrast, α-subunit production by sparsely granulated somatotroph adenomas is rare and, when present, much lower in quantity. Our studies confirm that densely and sparsely granulated somatotroph adenomas represent separate entities.