Phenylketonuria (PKU) is a heterogeneous and autosomal recessive metabolic disorder that is mainly caused by mutations in the hepatic phenylalanine hydroxylase (PAH) gene. This study was designed to identify PAH mutations within exons 6, 7, and 10-12 in PKU patients from southwest Iran. Forty Iranian patients with clinical and biochemically confirmed PKU were enrolled. The exons were sequenced directly and 13 different mutations were identified including I224T, S231P, R176X, c.592_613del22, R243X, R252W, R261Q, Y356X, V388M, IVS10-11G>A, IVS11+1G>C, IVS11-2A>G, and Q375R, which were associated with 23 genotypes. A novel sequence variant, Q375R (c.1124A>G), was detected in exon 11. In one patient, a typical genotype with more than two mutations (R243X/S231P/S231P) was found. Seven different polymorphisms and three new variants were also detected in intron regions of PAH.