Few studies of the frequency of chromosomal aberrations in an unselected material of mildly mentally retarded children have been published. The present paper summarizes the chromosomal abnormalities in children with mild mental retardation (MMR) in Västerbotten in the northernmost part of Sweden. Chromosome analyses were carried out by routine methods. In addition, children whose MMR had no clear diagnosis or cause were investigated as regarding X-chromosomes with a fragile site and selected cases with banding techniques. Every mentally retarded child born between 1959 and 1970 in the county of Västerbotten was traced. Out of a total number of 40,871 individuals, 171, i.e. 4.2 per 1,000, were found to be mildly mentally retarded. Chromosomal aberrations were seen in 11.9% of the children with MMR, compared with 39.1% of the 161 children with severe mental retardation (SMR) in the same population. The proportion of cases with mental retardation of unknown etiology is high, especially amongst those with MMR. The use of high resolution banding and other modern cytogenetic methods should reduce this figure.