Giant prolactinoma is a rare subset of macroadenomas. Limited studies demonstrated which therapy could be successfully used in the first-line therapy of giant prolactinoma. We presented a case with a 54x40x40 mm pituitary adenoma and optic chiasmatic compression with left sphenoid sinus invasion. The tumor caused a loss of visual field of the right side. Cabergoline treatment was started with dose of 1.5 mg/week. Fifteen days later, the clinical visual acuity examination showed a significant improvement in the patient with visual field defect. After the five years follow-up magnetic resonance imagining showed reduction of the adenoma size (17x12 mm) was significant. Our findings suggest that, cabergoline can be used as a first-line therapy in giant prolactinomas because tumoral shrinkage without a surgical procedure and rapid improvement in visual field defect is achieved with this medical treatment.