The mu-opioid receptor (MOR1) is a target of endogenous and exogenous opioids and plays a pivotal role for anesthesia and analgesia. Variations in the 5' flanking sequence of the mu-opioid receptor gene may influence transcriptional regulation and ultimately alter protein expression of MOR1. In the present study we investigated the influence of eight single nucleotide polymorphisms (SNP) within the mu-opioid receptor promoter on promoter activity and evaluated the frequencies of the relevant SNPs in 700 patients under opioid medication. Reporter-gene-constructs were created by means of PCR and site directed mutagenesis, testing eight SNPs previously described. The neuroblastoma cell line SHSY5Y was used for transfection and promoter activity was estimated by luciferase activity. Of the eight reporter gene constructs employed to test genomic variations, two produced a significant change in luciferase activity when compared to wild-type constructs. The G-554A variation located within a known NFkB binding element resulted in a decreased activity whereas the A/G base exchange at position -1320 showed an increased luciferase activity. This particular variant generated a myeloid zinc finger (MZF1) cis-acting element known to impact transcription. The allele frequency of the -1320G variant was 0.21% in 700 Caucasian patients under opioid medication in contrast to 9.1% reported previously in drug addicted African Americans. Because of this unexpected low frequency an association analysis to opioid requirements and effects of mu-opioid receptor agonists was not feasible. In conclusion, transcriptional regulation of MOR1 is modified by two genetic variations at positions -554 and -1320 of the mu-opioid receptor promoter. Individuals presenting these variations may have an altered level of MOR expression. A possible association of these genomic variants on efficacy and side effects of opioid treatment in different ethnic groups has to be elucidated.