Cigarette smoking can induce CYP1A1 in the lung. Induction requires the aryl hydrocarbon receptor (AHR) and aryl hydrocarbon receptor nuclear translocator (ARNT) proteins. Lung samples from seven of 75 Finnish patients who smoked until the time of surgery exhibited absent or low levels of CYP1A1 protein, mRNA and enzymatic activity, suggesting that these individuals might be genetically non or poorly inducible for CYP1A1. All seven lung samples expressed normal levels of AHR mRNA and ARNT mRNA, indicating that they did not carry inactivating polymorphisms in the 5′ upstream regulatory regions of these genes. Sequencing of cDNAs encompassing the complete coding regions ofAHRandARNTidentified a previously known codon 554 polymorphism inAHR, which was present in the homozygous state in one individual. This polymorphism, which leads to an amino acid substitution, has previously been reported either to have no effect or to enhance CYP1A1 induction. Previously unreported silent single nucleotide polymorphisms were identified in codon 44 ofAHRand codon 189 of ARNT. 1500 bp of genomic sequence from the 5′ upstream regulatory sequence of theCYP1A1gene was also sequenced in the non-inducible individuals. A nucleotide substitution polymorphism at position –459 was detected in the heterozygous state in two individuals. This polymorphic site does not reside in any known regulatory sequence. The completeCYP1A1coding sequence and intron/exon boundaries were then sequenced. None of the non or poorly inducible individuals exhibited any polymorphisms, either homozygous or heterozygous compared to representative inducible individuals or the previously publishedCYP1A1sequence. Thus, no polymorphisms in theAHR, ARNTorCYP1A1genes were identified that could be responsible for the non/low inducibility phenotype observed.