Reciprocal translocations are one of the most frequently observed structural chromosome abnormalities. They are defined by a segment exchange between two non-homologous chromosomes. A great number of different translocations exist since any chromosome can be involved in the translocation and the position of the breakpoint can vary. Though generally silent these translocations can be expressed in the form of reproduction failure or, more seriously, as offspring showing mental retardation/malformation syndromes. Since the risk of malformation varies from one translocation to the next, genetic counselling and prenatal diagnosis strategies should be adopted to suit the particular malformation risks of each individual translocation. This is currently not the case. Different prediction methods (for the most probable mode of unbalance at birth, the risk of unbalance at term) are presented. A computer system, called Reci-Conseil brings these different functionalities together to create a new aid for genetic counselling. The data base on which it is founded (approx 2000 families) offers interesting perspectives for genomic mapping of partial trisomies and monosomies.