Tobacco smoking is associated with increased prevalence of various diseases. In this study, we have used an indirect enzyme-linked immunosorbent assay (ELISA) to investigate the specific IgG response to pigeon serum and its relationship to tobacco smoking. In a large group of pigeon fanciers the development of pigeon breeder's disease and some of the clinical features were also investigated. We found a sensitization rate of 31.9%, a lower degree than that found by other authors, in spite of using a less specific antigen. We also found that fanciers who are current cigarette smokers had significantly lower levels of IgG antibody to pigeon serum (P less than 0.001). Finally, 11.9% of the population had chronic bronchitis as their only manifestation of possible pigeon-related disease. 19.6% of the sensitized fanciers had chronic bronchitis, were non-smokers and had no other risk factor for developing chronic bronchitis.