We present the case of a 61-year-old female who initially presented aged 41 years with a two-year history of infertility and galactorrhoea. She had an elevated prolactin level of 1258 mIU/L (29-513 mIU/L) with an otherwise unremarkable pituitary screen. MRI demonstrated a 1.0 x 0.6 x 0.6 cm left-sided pituitary adenoma. Weekly cabergoline 0.5 mg was commenced, which was continued for three years before being ceased and she was subsequently lost to follow up. Twenty years later, she re-presented with an eight-year history of increased hand and feet size, increased jaw size, and associated arthralgias. She had been commenced on anti-hypertensive therapy in the preceding two years, but denied diabetes, excessive sweating, skin tags, headaches, diplopia or visual loss. Investigations revealed an elevated IGF-1 level of 99 nmol/L (5-32 nmol/L) with preservation of the remaining pituitary axes, and in particular a normal prolactin level of 321 mIU/L. MRI of the pituitary demonstrated a locally invasive 2.3 x 4.3 x 2.8 cm pituitary mass with typical inferolateral expansion, eroding into and replacing the sphenoid sinus and clivus, invasion into the left cavernous sinus with lateral extension to the left internal carotid artery, and enveloping 180 degrees around the right internal carotid artery. The patient underwent endoscopic endonasal resection of the pituitary macroadenoma. Histology demonstrated a sparsely granulated somatotroph adenoma, with no additional pituitary hormone staining. Six months post-operatively the IGF-1 levels had normalised to 28 nmol/L and follow up imaging demonstrated near complete resection of the tumour with only a rind of heterogeneous enhancement remaining, and normal appearance of the remaining pituitary gland and stalk. In summary, our patient was diagnosed with a locally invasive isolated-staining somatotroph macroadenoma 20 years after her initial presentation with symptomatic hyperprolactinaemia, despite the absence of lactotroph staining and normal IGF-1 levels at initial presentation.