Background and Case Presentation: Cystic prolactinomas correspond to = 1% of all prolactinomas and its association with a giant pituitary macroadenoma in a woman has been seldom reported. A 43-year old woman was evaluated for a suprasellar tumor. She had absence of menses since 2 years. Three months before she started to have progressive headache. She also referred asthenia, adynamia, fatigue, dry hairiness, decreased libido, vaginal atrophy, and chronic constipation. Hair in armpits and pubic area was insignificant and osteotendinous reflex relaxation phase was slow. Galactorrhea was absent and presented bitemporal hemianopsia. Diagnostic and Therapeutic Approach: MRI scan revealed a 5 x 3.5 cm suprasellar mass and a 8.1 x 3.6 cm cystic component (Figures 1-3). Cortisol level 2.2 mg/dl, FSH and LH were low, estradiol < 0.05 pg/mL, IGF-1 normal and a serum prolactin of 125 ng/mL. Serial dilutions 1:10 and 1:100 were made. Diluted prolactine was 12,500 ng/mL (hook effect). Treatment with cabergoline, prednisone and levothyroxine were initiated. At two months follow-up prolactin serum levels decreased to 32.1 ng/mL. Discussion and Conclusion: A systemic research on PubMed, Medline, Embase and MedConsult with the search criteria: “Giant Cystic Prolactinoma”, “prolactin”, “prolactioma and “hook effect” was made. Due to the clinical signs of hypogonadism, the image in the MRI and the low/moderate high serum prolactin levels serial dilutions of prolactin were made. Serum prolactin assays can greatly underestimate extremely high levels of hormone, the so called “hook effect”. In present case, a high level of suspicion along with low/moderate prolactin levels and the MRI image consistent with a pituitary adenoma made us ask for diluted prolactin samples. If missed, this would have delayed the diagnosis and consequently would have changed the treatment plan and prognosis of the patient.