Obesity and overweight have been associated with an increased risk of asthma in children as well as adults. The association between atopy and body mass index (BMI) is less clear. It has also been suggested that the link between a high BMI and asthma could be a recent phenomenon. The objective of this study was to assess whether the association with BMI differed between allergic rhinoconjunctivitis and asthma and if these associations have changed over time. The Swedish Military Service Conscription Register was linked to the Register of the Total Population and the Population and Housing Censuses. Asthma (with and without allergic rhinoconjunctivitis) and allergic rhinoconjunctivitis at conscription were analysed in relation to BMI for 1,247,038 male conscripts in successive cohorts born between 1952 and 1977. Obesity was associated with asthma without allergic rhinoconjunctivitis, adjusted OR 1.53 (95% CI 1.43-1.63), and with asthma with allergic rhinoconjunctivitis, adjusted OR 1.34 (95% CI 1.20-1.50), but not with allergic rhinoconjunctivitis, OR 1.00 (95% CI 0.97-1.03) after multivariate analyses with adjustments for confounders. The odds ratios were similar in three successive cohorts (conscripts born in 1952-1961, 1962-1971 and 1972-1977). Underweight was associated with a slightly increased risk for all three conditions. The increased risk of asthma in young Swedish men with obesity has remained unchanged over a period of three decades.