Hamartoma of the tuber cinereum is a well-recognized cause of central precocious puberty. We report three patients with an isodense, nonenhancing mass within the interpeduncular cistern identified by CT. In a fourth patient, the CT scan was normal. MR imaging was obtained in all cases and demonstrated a sessile or pedunculated mass of the posterior hypothalamus arising from the region of the tuber cinereum. The smallest mass was 2 mm in diameter and was found in the patient in whom the CT scan was normal. The signal intensity of the masses was generally homogeneous and isointense relative to gray matter on T1- and intermediate-weighted images, and hyper-intense on T2-weighted images. MR imaging accurately diagnoses hypothalamic hamartomas, identifies small hamartomas of the tuber cinereum more sensitively than CT does, and provides optimal imaging for serial evaluation while the patient is being treated medically.