Gonadotroph adenomas resected from the pituitaries of 8 males and 8 females were studied by transmission electron microscopy and morphometry in order to establish whether they exhibit sexual dimorphism. Random photographs were taken and the following parameters compared: (1) cellular volume, (2) nuclear volume, (3) cytoplasmic to nuclear ratio, (4) volume density of endoplasmic reticulum, (5) volume density of Golgi bodies, (6) volume density of mitochondria, (7) volume density of secretory granules, (8) volume density of lysosomes, (9) diameter of secretory granules, (10) surface to volume ratio of secretory granules, (11) surface to volume ratio of Golgi bodies, (12) surface to volume ratio of nuclei, (13) surface to volume ratio of cells. The results showed that gonadotroph adenomas of males differed significantly from those of females in cell size, cell shape, and electron microscopic features of Golgi apparatus. It remains to be established whether differences in the morphology of non-neoplastic gonadotrophs of males and females can explain the development of morphologically distinct adenoma cells between the sexes.