We describe a young female patient with giant invasive sellar and suprasellar tumor and modest elevation of prolactin to 165 ng/ml (normal range 3-29). A diagnosis was made of non functional pituitary adenoma with stalk effect, causing moderate prolactin elevation. A surgery for the removal of the tumor was advised but the patient declined. Treatment with a dopamine agonist was not offered. The patient presented 2 years later with deterioration of her vision and serum prolactin of >16000 ng/ml. Debulking transsphenoidal surgery was performed. The staining of tissue confirmed prolactinoma. Medical treatment with bromocriptine was initiated. We believe that the discrepancy between the 2 values of serum prolactin, is most probably caused by a hook effect in the initial prolactin assay. The mechanism of the hook effect and its occurrence with prolactin immunoassays and methods to eliminate this effect is discussed. Hook effect needs to be suspected in every patient with a giant pituitary or parasellar mass and serum prolactin <200 ng/ml. Assaying a diluted serum will usually unmask this phenomenon and allow accurate diagnosis and management.