Mental retardation (MR) is a heterogeneous condition, affecting 1-3% of general population. Identification of chromosomal abnormalities (CAs) associated with mental disorders may be especially important given the unknown pathophysiology and the probable genetic heterogeneity of MR. In this study, we aimed to evaluate karyotype results of 772 MR cases, retrospectively. For karyotyping, standard lymphocyte culturing and GTG banding methods were done. For analysis, cytovision software was used. In result, out of 772 MR cases, 87 cases showed abnormal chromosomal constitutions (11.3%), and normal karyotype results were detected in 88.7% of all patients. Numerical and structural CAs were detected in 2.6% (20 of 772) and 8.7 (67 of 772) of cases, respectively. This study revealed Down syndrome as the most common chromosomal abnormality (1%). The ratio of X chromosome monosomy was 0.6%. In conclusion, patients with MR should be routinely karyotyped. Interesting CAs we found, may harbor important genes for MR and give important tips for linkage in possible genome scan projects.