The psychomotor and somatic development from early childhood into adult life is described in a man with 46,XY,r(8)/46,XY mosaicism. The ring chromosome 8 appeared to be of normal length on G-banding, but terminal deletions on 8q and 8p were detected with FISH and CGH. By STR marker analysis the 8p deletion proved to be quite large, at least 6.74 Mb, while the 8q deletion was small, around 2.5 Mb. The haplotype analysis also demonstrated that the r(8) originated from a maternal chromosome 8, and that cells with normal male karyotype resulted from monosomy 8 rescue after loss of the ring 8, i.e. a mitotic duplication of the paternal chromosome 8. The patient has a mild phenotype with no malformations and mild mental retardation, also compared to other ring chromosome 8 patients. His clinical condition has remained stable for the last 20 years.