Seven hundred mentally retarded patients attending the outpatient clinic of the Human Genetics Department, Medical Research Institute were assessed to estimate the frequency of chromosomal anomalies and biochemical defects present. The group included 355 males and 345 females. Chromosomal anomalies were found in 154 patients [22%] ; numerical autosomal anomalies were detected in 113 patients [16.1%], with trisomy 21 being the most common [107 patients, 15.3%], structural chromosome anomalies were present in 23 patients [3.3%], sex chromosome anomalies were detected in 4 patients 10.6%]. Fourteen patients were fra [X] +ve constituting 3.9% of the male patients included in this study. Inborn errors of metabolism were detected in 40 patients [5.7%]. Twenty patients [2.85%] had aminoacidopathies; 10 patients [1.43%] had phenylketonuria, 4 [0.57%] had alkaptonuria, 2 [0.28%] had homocystinuria and 4 [0.57%] had generalized aminoacidurias and twenty patients had lysosomal disease; 15 patients [2.14%] had mucopolysaccaridosis, 1 patient [0.14%] had mucolipidosis and 4 patients [0.57%] had sphinoglipidosis.