Rostral midbrain atrophy in progressive supranuclear palsy (PSP) is detected by mid-sagittal plain magnetic resonance imaging (MRI). The shape of the atrophy looks like the bill of a hummingbird (hummingbird sign). We studied this sign to elucidate the nature of midbrain atrophy in PSP. Eight patients with PSP, 12 with Parkinson's disease (PD), and 10 normal controls were studied. Using mid-sagittal plain MRI, we measured the rostral and caudal midbrain tegmentum (MT), superior and inferior colliculus, pontine base, and tegmentum. We compared the length of the interpeduncular fossa, which is posterior to the mammillary body, to the diameter of the midbrain tegmentum. The multiple comparison method was used for the statistical analysis. The hummingbird sign was demonstrated in all of the PSP patients studied, and it was not observed in PD patients nor in normal controls. The hummingbird sign in the PSP patients was due to the atrophy of the midbrain tegmentum (rostral and caudal) and to a relative increase in the length of the interpeduncular fossa over that of the anteroposterior diameter of the midbrain tegmentum. The hummingbird sign, which represents the atrophy of the rostral midbrain tegmentum, strongly suggests the involvement of the rostral interstitial nucleus of the medial longitudinal fasciculus in patients with PSP. Demonstration of a hummingbird sign on MRI is thought to be useful for a diagnosis of PSP.