Introduction: Knowledge of IgE-verified allergy in young adults is limited as most studies are based on self-reported data. Allergic heredity is important in allergy development in early life, but less is known about the hereditary component later in life. The aim was to investigate IgE-verified and self-reported allergy and asthma at 20 years of age in association to parental allergy and environmental factors. Methods: In total, 281 individuals born into the cohort of well-characterized parents regarding allergic disease were followed to 20 years of age. The participantswere categorized by parental allergy and examined regarding allergic diseases (IgE sensitization and allergic symptoms) at 2, 5, 10, and 20 years of age. FeNO wasmeasured at 10 and 20 years. Results: In total, 45% of the study participants were allergic, with twice as many self-reported cases at age 20. Rhinitis was key to distinguishing confirmed allergy from self-reported. Having two allergic parents and increased FeNO were associated with an increased prevalence of allergic disease at 20 years. From a longitudinal perspective, rhinitis increased from childhood to young adulthood, in all heredity groups. Conclusion: In this longitudinal study, we have shown that two allergic parents as well as increased FeNO levels seem to be of importance for being allergic at 20 years old. Selfreported allergy was overreported - a result that should be considered in future survey-based reports on allergic diseases.