It is well known that individuals with cerebral palsy (CP) are subject to higher mortality than the general population but the causes of this have not been systematically analysed. This study investigates mortality in a Californian population of 45,292 individuals with CP, 4028 of whom died during the 1986 to 1995 study period. The aims of this study were to identify diseases that may be causally linked with CP, and diseases whose diagnosis and/or treatment need improvement. Overall, the standardized mortality ratio (SMR) was 8.4. Mortality from breast cancer was three times that of the general population, suggesting poorer detection and/or treatment. The dramatic elevation of mortality due to brain cancer, especially in children (SMR=24), raises the possibility of a link between this and CP. Cause of death was non-specific in some individuals. Therefore, these SMRs are conservative. As expected, SMRs due to respiratory diseases were very high but, contrary to anecdotal reports, such diseases did not account for most deaths. High SMRs were also found for diseases of the circulatory and digestive systems. Finally, a marked elevation of deaths was due to external causes, including drowning and being hit by motor vehicles.