Allergic diseases such as atopic asthma are believed to have their origins in early life1 but the precise mechanisms and timings of the relevant immunoregulation continue to be a focus of research. One area of investigation has been the role of vaccinations in Th cytokine regulation.1 Epidemiological studies investigating the potential of BCG, a potent immunomodulator, to reduce the risk of atopic diseases report conflicting results.2 A Manchester study (MANCAS), using a cohort of children all born in the same hospital in the mid 1990s identified a lower risk of wheeze for children given neonatal BCG.3 Data analysis for a follow-up study 6 years later when the cohort was aged 13–17 yrs has just been completed. Using the same definitions for wheeze and asthma as the first study there was no difference in prevalence of wheeze or asthma between BCG vaccinated and non-vaccinated adolescents (Abstract P176 table 1). Significance tests between the studies were not performed because some participants responded only to one of the two studies. A Medical Research Council study4 in the 1950s investigating the effectiveness of a vaccination programme to prevent tuberculosis identified a progressive decrease in the efficacy of BCG in successive 5 year periods with 80% efficacy 5 years post vaccination reducing to 59% 10–15 yrs after vaccination. If the decrease in efficacy of BCG modifies its ability to protect against atopy it may be that the conflicting results in studies investigating BCG and atopy have occurred because the protection afforded by BCG is limited to within a timeframe after vaccination.