There is currently little and conflicting evidence concerning the existence of socio-economic inequalities in cerebral palsy prevalence, or the extent to which this is influenced by socio-economic inequalities in low birthweight, a strong risk factor for cerebral palsy. The study is based on 753 children registered with cerebral palsy, resident in the former Oxford Regional Health Authority area and born in the years 1984-90. Two population definitions were used: 1. Children with cerebral palsy resident at birth in the area, with resident births as denominator, 2. Children with cerebral palsy resident at age 5 in the area, with children of ages 1-7 resident in the area in the 1991 census as denominator. Children with cerebral palsy and all births/children were classified according to the Carstairs area deprivation index (grouped into quintiles) of their ward of residence. The prevalence among residents at birth varied from 2.08 per 1000 births in the most affluent quintile to 3.33 in the most deprived quintile (trend P < 0.001). Although there was a tendency for children to move to more affluent areas during early childhood, the socio-economic gradient was similar at age 5. A greater proportion of births in the more deprived quintiles were of low or very low birthweight, the proportion rising from 5.6% in the most affluent quintile to 8.2% in the most deprived. Within the normal birthweight category there was a trend for higher prevalence of cerebral palsy in more deprived quintiles, from 1.29 per 1000 in the most affluent quintile to 2.42 in the most deprived quintile (trend P < 0.001). Within the low birthweight and very low birthweight groups, separately or combined, there was no evidence of any relationship between cerebral palsy prevalence and deprivation. We estimate that up to 17% of cerebral palsy cases might be "preventable" in terms of the reduction to be expected if the whole population had the rate of cerebral palsy of the most affluent quintile. Although the strong socio-economic gradient for cerebral palsy was restricted to the normal birthweight category, we estimate that two-thirds of the excess cases in the population associated with greater socio-economic deprivation were normal birthweight cases, and one-third were low birthweight cases owing to the greater prevalence of low birthweight in more deprived populations. The pattern of socio-economic inequalities should be further explored in other regions, and should be taken into account in aetiological research, and in the effective delivery and evaluation of services.