Rationale: Preeclampsia reflects an unusual increase in systemic inflammation during pregnancy. Objectives: We studied associations between preeclampsia and asthma, allergy, and eczema in Copenhagen Prospective Studies on Asthma in Childhood(2000) (COPSAC(2000)) and in national registries. Methods: COPSAC(2000) is a high-risk birth cohort of 411 Danish children. Asthma, allergy, and eczema were diagnosed prospectively, and lung function measured at age 1 month and 7 years. Sensitization was evaluated at age 6 months, 18 months, 4 years, and 6 years by skin prick tests and IgE measurements. The register-based cohort included 1.7 million children from Danish national registries in the 35-year period 1977-2012. Children born to mothers with preeclampsia were analyzed regarding risk of asthma, allergy, and eczema. Measurements and Main Results: In the COPSAC(2000) cohort, 5.6% (n = 23) were diagnosed with preeclampsia. Preeclampsia was associated with increased risk of treatment with inhaled corticosteroids at age 7 years (adjusted odds ratio, 4.01 [95% confidence interval (CI), 1.11-14.43]; P = 0.0337), increased bronchial responsiveness to methacholine (adjusted beta-coefficient log-mu mol, -0.80 [95% CI, -1.55 to -0.06]; P = 0.0348), and allergic rhinitis (adjusted odds ratio, 4.83 [95% CI, 1.58-14.78]; P = 0.0057) in the 7-year-old children. Furthermore, the children had an increased risk of sensitization to both aeroallergens and food allergens, and increased amount of total IgE during childhood. In the registry-based cohort, 3.7% (n = 62,728) were born to mothers with preeclampsia. Preeclampsia was associated with increased risk of asthma, eczema, and aeroallergen and food allergy, especially pronounced after a duration of preeclampsia of 14 days or more. Maternal asthma increased the risk of preeclampsia. Conclusions: Preeclampsia is a shared prenatal risk factor for asthma, eczema, and allergy in childhood pointing toward in utero immune programming of the child.