A study on 1314 children with mental retardation (MR) without an obvious environmental cause was carried out at Bangalore, Bombay, Delhi and Lucknow to determine the extent and pattern of genetic causes of mental retardation in different parts of India. In all, 42.3 per cent patients had mild, 25.3 per cent moderate, 19.2 per cent severe and 13.1 per cent profound mental retardation. Among 1314 patients, the chromosomal anomalies were found in 23.7 per cent, metabolic defects in 5.0 per cent and an identificable genetic syndrome in 11.6 per cent of the patients. In the remaining 59.7 per cent patients, no known genetic cause could be identified. However, 66.5 per cent of these patients had one or more of the following conditions: (i) congenital malformation with or without neurological deficit, (ii) history of consanguinity, (iii) positive family history of mental retardation or (iv) a positive screening test but without a confirmed diagnosis of metabolic defect (suggesting that there may be additional unidentified genetic causes of mental retardation).