The total population of 11,865 children of compulsory school age resident on the Isle of Wight was studied to determine the prevalence of epilepsy, cerebral palsy, and other neurological disorders. With the use of reliable methods, children selected from screening of the total population were individually studied by means of parental interviews and questionaries, neurological examination and psychiatric assessment of each child, information from school teachers, and perusal of the records of hospitals and other agencies. The association between organic brain dysfunction and psychiatric disorder was studied by comparing the findings in the children with epilepsy or with lesions above the brain stem (cerebral palsy and similar disorders) with those in (1) a random sample of the general population, (2) children with lesions below the brain stem (for example, muscular dystrophy or paralyses following poliomyelitis), and (3) children with other chronic physical handicaps not involving the nervous system (for example, asthma, heart disease, or diabetes).Psychiatric disorders in children with neuro-epileptic conditions were five times as common as in the general population and three times as common as in children with chronic physical handicaps not involving the brain. It was concluded, on the basis of a study of factors associated with psychiatric disorder, that the high rate of psychiatric disorder in the neuro-epileptic children was due to the presence of organic brain dysfunction rather than just the existence of a physical handicap (though this also played a part). However, organic brain dysfunction was not associated with any specific type of disorder. Within the neuro-epileptic group the neurological features and the type of fit, intellectual/educational factors, and socio-familial factors all interacted in the development of psychiatric disorder.