The etiology of attention deficit disorder and hyperactivity (ADDH) is controversial because both biologic and social factors have been postulated. To study such factors, we undertook a referral study based on an entire cohort (N = 6,950) of children born in Jerusalem in 1976. Of 479 children referred for learning disabilities, hyperactivity, and behavioral problems, 381 were available for study. Information regarding obstetric, developmental, and family histories was obtained by a detailed, structured interview. A DSM-III-based questionnaire for ADDH was completed by parents and teachers. Each child underwent neurologic examination and 133 had IQ testing. Of these children, 145 fulfilled the criteria for ADDH. Our results revealed a number of significant familial-genetic factors: boys outnumbered girls by 3:1, 30% of ADDH children had siblings with learning disabilities (P less than .001); and ADDH children clustered within families of North African descent (P less than .001). The only significant developmental factor was delayed language development. Of numerous pre- and peri-natal factors investigated, only intrauterine growth retardation was significantly associated with ADDH. No correlation was found between ADDH and IQ, parental age, years of education, profession, and language spoken at home. Our study supports the hypothesis that familial-genetic factors are contributory to ADDH.