The purpose of this study was to determine whether the MR findings of idiopathic pituitary dwarfism correlated with a clinical history of perinatal abnormalities and abnormal levels of pituitary hormones. MR examinations of 18 patients with pituitary dwarfism were performed; these patients were divided into two groups: those with ectopic posterior lobes (group 1) and those with normal posterior lobes (group 2). Among the seven patients in group 1, MR showed hyperintense signal at the median eminence of the hypothalamus, which was regarded as the ectopic posterior lobe of the pituitary gland. The mean anteroposterior length and height of the pituitary gland in group 1 patients were significantly smaller than those dimensions in the normal control group; the pituitary stalk was not detected in three of seven patients. Six of seven patients were products of breech presentation with perinatal asphyxia. The peak serum growth hormone level was less than 5 ng/ml when assessed by insulin-induced hypoglycemia or the clonidine test. MR findings in the 11 patients with pituitary dwarfism and normal posterior lobes were normal except that the mean size of the pituitary gland was slightly smaller than that of normal controls. The clinical history of these patients was normal except for perinatal asphyxia in one case. Our findings in patients with pituitary dwarfism, with or without an ectopic posterior pituitary lobe suggest that the ectopic lobe, visualized as a bright spot at the median eminence of the hypothalamus, may be common when pituitary dwarfism follows perinatal anoxic/ischemic episodes.