Children with bilateral cerebral palsy (CP) born during 1989 to 1992 (n=346) to a geographically defined population were ascertained and followed up to age 5 years. The aims of the study were to monitor hip development by serial X-rays; to record gross locomotor development, aspects of physical management, and any hip-related orthotics or surgery; to learn more of the natural history of hip development in this condition; and to provide guidelines for a surveillance protocol for those clinically managing children with bilateral CP. Children were X-rayed at 18, 24, 30, 48, and 60 months. X-rays were taken in a standardized position and measured to record migration percentage, acetabular index, Sharp's angle, and Smith's diaphyseal ratios. Relationships between the measures were investigated and a natural history of the children's hip development suggested. At as early as 18 months, migration percentages were significantly greater than in the normally developing population, although no measured CP factor could be identified to be the cause of this early change. Two possible factors affecting early hip development are described, neither of which is clinically apparent: the first separates the bilateral CP population as a whole from the normal population; but there is an additional presdisposing factor affecting some children only which in the presence of the motor disorder, has a significant influence on early hip development and subsequent dysplasia. The study confirms that, when measured correctly, migration percentage is the best guide to hip surveillance and the need for treatment. It is suggested that all children with bilateral CP should be X-rayed in a standardized position at 30 months (corrected for gestational age).