We examined four polymorphisms in the mu opioid receptor gene in 282 Chinese heroin addicts from Sichuan Province, Southwest China and compared the allele and genotype frequencies to those in 258 normal controls from the same geographic region. Two of these polymorphisms (Ala6Val and Ser147Cys) were not polymorphic in the Chinese, with only Ala6 and Ser147 observed. The frequencies of the two other polymorphisms were significantly different from those observed in Caucasians, African Americans and Native Americans. The Asn40Asp and IVS2 + 691G/C polymorphisms did not differ significantly for allele (p= 0.16; p = 0.21), genotype (p= 0.32; p = 0.09) or haplotype frequencies (p= 0.24) between the Chinese heroin-addicted cases and normal controls. Similarly, we did not detect any association when the population was stratified by gender, route of administration (nasal inhalation and/or injection) and age-at-onset (above or below 25 years). This indicates that the mu opioid receptor is not likely to be a major genetic risk factor for heroin abuse in this population.