Sixteen patients out of 58 children with congenital hydrocephalus were retrospectively investigated in terms of their psychomotor development and morphological findings of the brain by magnetic resonance imaging (MRI), and morphological backgrounds of the brain in patients affected by "intractable hydrocephalus" were intended to be clarified. The 16 patients were primarily diagnosed as having aqueductal stenosis or communicating hydrocephalus during the era before introduction of MRI. On the basis of the rate of head growth, sixteen patients were divided into three groups; group-1: overt neonatal hydrocephalus (5 cases) ; group-2: rapidly progressive hydrocephalus with normal head circumference at birth (5 cases); and group-3: slowly progressive hydrocephalus with its onset after the neonatal period (6 cases). Psychomotor development of the 16 cases was analyzed by using various methods including the Tanaka-Binet intelligence test and WISC-R. Four cases in group-1 and all cases in group-2 were found to have IQ or DQ below 50, while the other remaining cases were found to have IQ of 82 in group-1 (one case) and IQ ranging from 67 to 113 in group-3. MRI performed in those 9 cases with IQ below 50 revealed various morphological abnormalities of the brain. Those abnormalities were characterized by disturbance of organogenesis and/or histogenesis of the brain dating from the first 20 weeks of fetal life. It is strongly suggested that in the pathogenesis of intractable hydrocephalus developmental malformation of the brain occurring during the first 20 weeks of gestation are involved. Functional and morphological impairments of the brain possibly caused by progression of fetal hydrocephalus and delay of CSF diversion before and after birth seem important in the pathogenesis of intractable hydrocephalus.