A register of children with cerebral palsy born in the period 1966-77 to mothers resident in the Mersey region was compiled from several different data sources. There were 685 cases, with a male:female ratio of 1.4:1. The birth prevalence of cerebral palsy ranged from 1.18 to 1.97 per 1000 live births each year, with a mean of 1.51 per 1000 live births. There was no discernible trend in overall prevalence, but there was a highly significant upward trend in the prevalence of cerebral palsy among low birthweight infants, which was evident for all the major types of cerebral palsy. It is postulated that this is due to improved survival rather than a real change in incidence. Alternatively, because the register was compiled from health service records, a diagnostic access bias from improved follow up of low birthweight infants may have led to an apparent increase in prevalence.