We compare the age and sex structure of China's 2000 population census to an estimate of that structure derived from a projection from the 1990 census. Based on China's own official estimates of demographic change, our intercensal analysis indicates a shortfall in enumeration of more than a quarter of all children under age 5 and an eighth of those between 5 and 9, a total of nearly 37 million children missing in the 2000 census. We show that the shortfall is primarily due to underreporting of children in the census. Sex differences in child underreporting were fairly minor. Child underreporting in China is not unprecedented, but child underreporting rates in 2000 were about triple those of previous censuses. We attribute the increase primarily to policy changes beginning in the early 1990s that held officials at all jurisdictional levels personally responsible for enforcing birth quotas.