Background: Suggestions that immunisation influences allergic disease risk, ei ther positively (pertussis) or negatively (BCG)-are of concern for vaccination policy. Aims: To determine whether DTP, MMR, and BCG vaccination in infancy infl uenced hay fever risk. Methods: Case-control study of 7098 hay fever cases and controls, within two primary care databases. One control per case was matched fo r practice, age, and sex. Odds ratios (OR)were derived using conditional logisti c regression. Results: Compared to those completing in month 5 (base group) (39. 3%), DTP unvaccinated children (4.3%) had a similar risk of hay fever (OR = 0. 94, 95%Cl 0.73 to 1.23). However, those completing after 12months (4.2%) had a reduced risk (OR = 0.60, 95%Cl 0.45 to 0.76) compared to the base group. Compa red to those vaccinated in month 14 (base group) (29.5%), MMR unvaccinated chil dren (2.3%) had an OR of 0.79 (95%Cl 0.58 to 1.08). Completion of MMR after tw o years was associated with reduced hay fever risk (OR = 0.62, 95%Cl 0.48 to 0. 80) compared to the base group. The effects of late immunisation with DTP and MM R were independent. Those vaccinated with BCG by age 2 (2.4%) had an odds ratio of 1.28 (95%Cl 0.96 to 1.70). Adjustment for consulting behaviour, social fact ors, or sibship size did not alter these associations. Conclusions: Immunisation against DTP or MMR does not increase the risk of hay fever. The lower confidenc e limit for BCG vaccination contradicts the hypothesised protective effect. The reduced risk of hay fever among children immunised late may be explained by a th ird factor causing both postponement and reduced risk such as intercurrent febri le illnesses. Background: Suggestions that immunisation influences allergic disease risk, ei ther positively (pertussis) or negatively (BCG) -are of concern for vaccination policy. Aims: To determine whether DTP, MMR, and BCG vaccination in infancy of hay fever risk. Methods : Case-control study of 7098 hay fever cases and controls, within two primary care databases. One control per case was matched fo r practice, age, and sex. Odds ratios (OR) were derived using conditional logistic c regression. DTS unvaccinated children (4.3%) had a similar risk of hay fever (OR = 0.94, 95% Cl 0.73 to 1.23). However, these completing in month 5 (base group) (39.3% Compa red to those vaccinated in month 14 (base group) (29.5%), MMR unvaccinated chil dren (4.2%) had a reduced risk (OR = 0.60, 95% Cl 0.45 to 0.76) compared to the base group 2.3%) had an OR of 0.79 (95% Cl 0.58 to 1.08). Completion of MMR after tw o years was associated with reduced hay fev The effects of late immunization with DTP and MM R were independent. Those vaccinated with BCG by age 2 (2.4%) had an odds ratio (OR = 0.62, 95% Cl 0.48 to 0.80) compared to the base group. of 1.28 (95% Cl 0.96 to 1.70). Adjustment for consulting behavior, social fact ors, or sibship size did not alter these associations. Conclusions: Immunization against DTP or MMR does not increase the risk of hay fever. The lower confidenc e limit for reduced risk of hay fever among children immunized late may be explained by a third factor causing both postponement and reduced risk such as intercurrent febrile disease.