This project has established a population-based registry of children with cerebral palsy (CP) among births 1983-1985 in four San Francisco Bay Area counties. Elements completed include: (a) Study of demographic and medical characteristics related to the occurrence of cerebral palsy in a contemporary American population; (b) CP in twins. Twin pregnancies produced a child with CP twelve times more often than singleton pregnancies. Much but not all of this risk was related to the tendency of twins to be low in birthweight. Twins of unlike-sex pairs, necessarily dizygotic, were not at lower risk than twins of like sex pairs. If one twin died in utero, the surviving co-twin was more than 100 times more likely than a singleton to have CP. Twinning is increasing in developed countries, and is likely to contribute more children with CP. Paper published, a confirmation in another country completed and published and an extension in a population that permits evaluation of other neurologic outcomes (the NCPP) completed and submitted for publication; (c) a case control- study within the California population-based cohort was undertaken to examine whether in utero exposure to magnesium sulfate was associated with diminished risk of CP in children of birthweight <1500g. It was the finding is dramatic and potentially important: infants of women treated with magnesium had a substantially lower frequency of CP. This study, including careful scrutiny for potential confounding factors, has been completed and submitted for publication. In progress; (d) an International Collaborative Study of Childhood Neurologic Disorders in Multiple Births has been initiated and a colleague from Western Australia will spend 6 months in the NEB to work on these data, beginning December 1994; ( f) Low birthweight is an important risk factor for CP. A study of risk factors for CP in infants <1500 g, comparing survivors with CP with children of that birthweight who survived without CP is underway.