This paper will review genetic variations in the structure of three important candidate genes with varying effects in atopy and asthma that may have significant overall susceptibility associations in relation to the development of atopy and asthma. The three cytokine genes involved are interleukin (IL)-4, IL-9 and transforming growth factor beta1, and the nature of the polymorphisms may be related to significantly higher outputs of these cytokines in atopy and asthma.