A family is described in which a pericentric inversion (13) was discovered in the father after the birth of an abnormal baby. In a further pregnancy amniocentesis was carried out. The fetal karyotype showed a rec(13)dup p,inv(13)(p11q22). The fetus's abnormalities were similar to those observed in the first child. Family studies showed that a first cousin, mentally retarded, had a rec(13)dup q,inv(13)(p11q22) karyotype. In this family, the risk of occurrence of a recombinant in offspring of an inversion carrier could be as high as 40%.