SUMMARYA comprehensive genetic/diagnostic survey was undertaken at a special school for the mentally retarded involving 105 patients. Cytogenetic, biochemical and clinical investigations were undertaken to establish the contribution of the genetic factors to the problem of mental retardation. Apart from obtaining information about specific children, identifying families at risk, and providing genetic counselling in nearly 50% of cases, data was obtained which could be compared with other similar surveys. According to the aetiological grouping of the patients, 6.7% could be attributed to perinatal damage, 17.1% to chromosomal defects, 4.8% to biochemical disorders, 5.7% to other genetic causes, 12.4% to other prenatal damage, 1.9% to infections, and 51.4% to unknown causes. No individual with the marker X syndrome was found in this group.