A family with primary infertility and two members with mental retardation and subtle facial dysmorphism is described. In the two retarded persons chromosomal rearrangements (partial monosomy of chromosome 5 and partial trisomy of chromosome 7) were detected. One member of the family had died with major congenital malformations. Her fibroblasts had been stored and her chromosomes showed the inverse pattern (partial trisomy of chromosome 5 and partial monosomy of chromosome 7). It appeared that in familial mental retardation with or without congenital malformations FISH-techniques should be used to detect submicroscopic chromosomal aberrations, which are not detectable by routine chromosome studies.