Genetic analysis of 169 mentally retarded (MR) children from Madras, revealed chromosomal abnormalities in 17%. Down syndrome was the major chromosomal anomaly (24/169 = 14.2%). These included three cases of trisomy-21 mosaics, and one case of de novo Robertsonian translocation. MR children with chromosomal abnormalities were either mildly or moderately retarded. Syndromes with known etiology occurred in 3% of the MR cases. Microcephaly, neonatal anoxia, perinatal stress and pharmacological attempt for abortion were found to be important pathogenic factors associated with MR. Most of the microcephalics (11/169 = 6.5%) were severely retarded, whereas those associated with neonatal anoxia and perinatal stress were either mildly or moderately retarded. Birth-order effects were found only among Down syndrome patients. Segregation analysis of the three groups of proband families (viz. mild, moderate and severe MR) indicated that autosomal recessive mode of inheritance is compatible in moderate and severe MR proband families. The proportion of X-linked instances of MR is estimated to be about 22% of the cases.