Levels of cytokines are used for in-depth characterization of patients with asthma; however, the variability over time might be a critical confounder. To analyze the course of serum cytokines in children, adolescents and adults with asthma and in healthy controls and to propose statistical methods to control for seasonal effects. Out of n= 532 screened subjects, n= 514 (91.5%) were included in the All Age Asthma Cohort (ALLIANCE). The cohort included n= 279 children with either recurrent wheezing bronchitis (> 3 episodes) or doctor-diagnosed asthma, n= 75 healthy controls, n= 150 adult asthmatics and n= 31 adult healthy controls. Blood samples were collected and 25 μL of serum was used for analysis with the Bio-Plex Pro™ Human Cytokine 27-Plex Assay. Mean age, body mass index and gender in the three groups of wheezers, asthmatic children, and adult asthmatics were comparable to healthy controls. Wheezers (34.5%), asthmatic children (78.7%) and adult asthmatics (62.8%) were significantly more often sensitized compared to controls (4.5%; 22% and 22.6%, respectively). Considering the entire cohort, IL-1ra, IL-4, IL-9, IL-17, MIP-1α and TNF-α showed seasonal variability, whereas IL-1β, IL-7, IL-8, IL-13, Eotaxin, G-CSF, IP-10, MIP-1 β and PDGF-BB did not. Significant differences between wheezers/asthmatics and healthy controls were observed for IL-17 and PDGF-BB, which remained stable after adjustment for the seasonality of IL-17. Seasonality has a significant impact on serum cytokine levels in patients with asthma. Since endotyping has achieved clinical importance to guide individualized patient tailored therapy, it is important to account for seasonal effects.This article is protected by copyright. All rights reserved.