Autosomal recessive inheritance of non-syndromic mental retardation (ARNSMR) may account for approximately 25% of all patients with non-specific mental retardation (NSMR). Although many X-linked genes have been identified as a cause of NSMR, only three autosomal genes are known to cause ARNSMR. We present here a large consanguineous Turkish family with four mentally retarded individuals from different branches of the family. Clinical tests showed cognitive impairment but no neurological, skeletal, and biochemical involvements. Genome-wide mapping using Human Mapping 10K Array showed a single positive locus with a parametric LOD score of 4.92 in a region on chromosome 1p21.1-p13.3. Further analyses using polymorphic microsatellite markers defined a 6.6-Mb critical region containing approximately 130 known genes. This locus is the fourth one linked to ARNSMR.