After an overview of definitions of mental retardation and recent case law regarding mental retardation and the death penalty, this paper presents a study of factors associated with a mental retardation (MR) diagnosis among murder defendants. Subjects with a full-scale IQ≤70 (n=42) were compared with other pretrial murder defendants (n=228) referred for forensic evaluation over a 5-year period. Subjects with an IQ≤70 who were diagnosed with MR were compared with subjects with an IQ≤70 who did not receive this diagnosis. Female murder defendants were more likely to receive a diagnosis of MR (p=0.03). MR was also more commonly diagnosed in subjects with an Axis I cognitive disorder (p=0.018). Having an IQ≤70 was more common in subjects with a psychotic and substance use disorder (p=0.03) and did not necessarily lead to a diagnosis of MR in this subgroup. Implications for diagnosing MR among murder defendants are discussed.