The study sought to identify factors with consistent relationships with the prevalence of asthma and postulate causes for the increasing prevalence. Reduplication of prevalence surveys was conducted among children of similar ages (6–12 years) from the same area (Tainan City, Taiwan). A total of 7523 primary school children from 1993 and 7224 from 1997 participated in the study. The reported prevalence of diagnosed asthma increased from 6.46% in 1993 to 8.45% in 1997 (relative risk 1.31, 95% CI 1.16, 1.47). Among all environment- and heredity-related factors examined, only four showed consistent relationships with childhood asthma, and all appeared to be a related hereditary condition. Dander allergy is the only factor that increased correspondingly with the prevalence of childhood asthma; therefore, it seems to be the most likely key factor responsible for the increasing trend. The investigation of the interactive effects imposed by the environment- (dander exposure) and heredity-related (atopic sensitisation) factors are recommended for further studies.