Cytochrome P450 (CYP) 2C9 is responsible for the metabolism of a number of widely used drugs such as oral anticoagulants, oral antidiabetics and non-steroidal anti-inflammatory drugs. The CYP2C9 is a genetically polymorphic enzyme. The most common allele is CYP2C9*1, while CYP2C9*2 and CYP2C9*3 are the less-frequent variants. The activity of the enzyme encoded by either CYP2C9 *2 or *3 variant is lower compared with that of the CYP2C9*1. The metabolism of most of the CYP2C9 substrates decreases in varying degrees in subjects carrying the CYP2C9 *2 or *3 allele. The aim of this study was to investigate the frequencies of the major variants of the CYP2C9 in Saudi Arabians.