A new computer system for genetic counseling in reciprocal translocations is described. This system, namely RCPc (RCP counseling) is a knowledge base extracted from a data base called SCD (Structural Chromosome Data) which contains 1376 families carrying reciprocal translocations. RCPc gives key information for each translocation which allows an evaluation of the risk of unbalance at birth and a prediction of the characteristics of potential unbalances. This information could provide a useful basis for deciding whether a prenatal diagnosis is required and if so, the preferred sampling method i.e. amniocentesis or chorionic villus sampling (CVS).