The association of hyperprolactinemia over 100 ng/ml and a pituitary adenoma is usually diagnostic of prolactinoma. However type 3 pituitary adenomas can occur with similar serum prolactin values. We report a 31 years old woman that consulted due to headache and photopsiae. She had a serum prolactin level of 148 ng/ml and imaging studies showed a solid tumor that occupied the selar region and most of the left temporal fossa. The tumor was partially resected and the patient recovered her lost visual field. Light microscopy showed an acidophilic and in part chromophobe adenoma. Immuno-histochemistry was positive for prolactin and growth hormone. Electron microscopy disclosed features of a silent type 3 adenoma such as big cells with cytoplasmic prolongations, pleomorphic nuclei and a greatly developed rough endoplasmic reticulum. (Rev Med Chile 2001; 129: 295-8).