The diagnosis of hereditary retinal syndromes may be difficult for a physician because of their number and variability. A computer assisted diagnosis of these syndromes can be useful in such cases. We used an identification software (XPER) for this purpose. The data base contains more than 67,000 elementary data that enable us to define 115 hereditary retinal syndromes. The knowledge is not represented by description of typical cases or diagnostic procedure rules but by structured description of syndromes defined by the group of experts. This CAI software is characterised by specific optimising procedures, deductive algorithms and dissimilarities calculus and enables very fast diagnoses by limiting the number of complementary analyses and thus the cost of this research. This system is extensible and justifies all its conclusions, its user-friendly data representation makes it accessible for any physician even if he does not master computers. The selected pathology field seems very suitable to developing a computer assisted diagnosis system: many low frequency syndromes, meaningful precise diagnosis for genetic and professional counseling, therapeutic expectations due to progress in molecular genetics. According to the authors, HERETAIN is one of the largest computer assisted decision support systems in ophthalmology.