The smoking habit is influenced by culture, psychological traits and physical factors. Recent studies on genetic polymorphisms have demonstrated that functional polymorphisms pertaining to neurotransmitters may affect smoking behavior, as well as psychological parameters and diseases. Our recent study demonstrated the interleukin (IL)-1B gene encoding IL-1â, a pro-inflammatory cytokine, to be associated with smoking; Japanese with the IL-1B C-31T T/T genotype, an inflammation-prone trait, were less likely to be smokers than those with IL-1B C-31T C/C (J Epidemiol 2001;11:120-125). This indicates that genetically determined biochemistry may also be an important factor for smoking behavior. We have investigated this association in another population, 963 Japanese Brazilians (399 males and 564 females) aged 33-69 years from Curitiba, Mogi das Cruzes, and Mirandopolis in Brazil. Current smokers were 15.3% among males and 11.6% among females. The sex-age-adjusted odds ratio (OR) for the polymorphism in males was around unity. In females, the adjusted OR of being current smokers vs. non-current smokers was 0.68 (95% confidence interval, 0.32-1.45) for the T/T genotype relative to the C/C genotype, and the adjusted OR of being ever smokers vs. never smokers was 0.85 (0.46-1.58). Significant reduction in the OR was not observed for either males or females with the T/T genotype. Although the inconsistent result could be caused by random variation, effect dilution caused by incorporation of a group with a smoking-free culture is a possible reason for the apparent anomaly. The effect of dilution may thus have to be taken into consideration, especially for studies on the smoking habit and genetic polymorphisms.