In the transition from childhood to adulthood there is a reversal in the sex distribution of asthma from a male predominance to a female predominance. Studying these sex differentials may contribute to our understanding of the processes that initiate and perpetuate disease. In a matched case control study the OR of asthma in the offspring of mothers who used the oral contraceptive pill (OCP) prior to pregnancy was 1.16 (95% CI 1.06, 1.27). These were predominantly female, under three year’s old, non-atopic and not receiving medication for asthma. In a cohort study of 223, higher oestrogen levels in late pregnancy were associated with higher IFN-γ production in cord blood cells and a reduced risk of wheezing in the first 6 months of life only. In surveys of children aged 9-11 from 1989 to 2004 the male to female ratio narrowed significantly for wheeze (1.34 to 0.98:1 p<0.0002), asthma (1.74 to 1.02:1 p<0.0001), eczema (1.42:1 to 0.81:1 p<0.0001) and hay fever (1.46 to 0.93:1 p<0.0001). In two whole population datasets covering UK (CMR and GPRD), sex specific presentations for asthma and allergic rhinitis peaked in early adolescence for females and later for males. Eczema presentations peaked in infancy in both sexes but females predominated from 15-49 years. In 9-11 year old children examined in 1989 males were more likely to have wheeze (OR 1.63; 95% CI 1.15, 2.23) and current asthma (OR 2.80; 95%CI 1.72, 4.56). Of the 711 participants followed 10 years later males were twice as likely to lose their childhood wheeze (OR 2.03; 95% CI 1.11, 3.72) and have significantly less BHR than females (OR 0.65; 95% CI 0.48, 0.89). Higher tertiles of testosterone was associated with a 50% reduction in the risk of BHR (OR 0.49; 95% CI 0.25, 0.93). This is an exciting area for future research.