The psychomotor development of mentally retarded children (IQ less than or equal to 85) in their first year is compared here with that of healthy children on the basis of a 1-year birth cohort (1966) from northern Finland which comprised a total of 12 058 live-born children. Data on the age of reaching the main developmental milestones in the first year, standing without support, walking with and without support and speech at 12 months were obtained by means of information collected prospectively in the children's welfare centres for this purpose. Detailed information on mental retardation covering the age group 0-14 years was obtained from the school health authorities, the national hospital discharge register and other regional or national registers. The results show that mental retardation is associated with subnormal motor development in the first year, as this finding persists even after the exclusion of children with a CP syndrome. Children who could neither walk with support nor speak by the age of 12 months constituted a high-risk group for mental retardation, the risk being 14-fold higher in this group than among the other children. Even so, only 65.2% of the children with severe mental retardation and 38.2% of those with mild mental retardation fulfilled these criteria, demonstrating that psychomotor development in the first year can be apparently normal in these groups.