A female child had multiple anomalies and a long B-group chromosome. Her normal mother was a carrier of a reciprocal translocation, presumably between chromosomes No. 5 and No. 14. The proband's karyotype cannot be interpreted on the basis of usual meiotic events in the mother. The simplest explanation involves meiotic pairing of maternal translocation chromosomes in a quadriradial configuration, and two breaks to form the long B-group chromosome observed in the proband. As a result, the proband was trisomic for a segment of the long arm of chromosome No. 5. Unusual meiotic events in carriers of reciprocal translocations have important implications with respect to genetic risks for offsprings with congenital defects and prenatal diagnosis from cytogenetic analysis of cultured amniotic cells. Many questions concerning the behavior of human translocation chromosomes at meiosis still remain to be answered.