The cytogenetic data of a large scale study in an institutionalized population of 1991 moderately and severely mentally retarded patients are presented. In 21.3% of these patients a chromosomal aberration was diagnosed (14.9% Down syndrome patients and 3.5% "other chromosomal anomalies"). A fragile X screening was performed in 354 males, and was found to be positive in 57 (16.1% positive). Different chromosomal variants were present in another 6% of this population. Detailed results are presented on the different groups of chromosomal aberrations.