A complex four-break rearrangement between chromosomes 4 and 13 was ascertained in a 10-year-old mentally retarded girl. The rearrangement was inherited from the phenotypically normal mother, who had an inverted insertion of part of the long arm of chromosome 4 into the long arm of 13 and, in addition, a pericentric inversion of the deleted 4. Meiotic crossing-over between the normal and the inverted 4 resulted in a recombinant chromosome 4, which was inherited by the proband, together with the 13/4 insertion. In this way the proband became monosomic for 4q35→qter and trisomic for 4pter→4p15, but she showed only minor physical malformations, as compared with other reports on the trisomy 4p syndrome. The cytogenetic findings were difficult to describe using the ISCN nomenclat