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Flavie Perrier,1 Reza Ghiasvand,2,3 Simon Lergenmuller,1 Trude E Robsahm,3 Adele C Green,4,5 Kristin B Borch,6 Torkjel M Sandanger,6 Elisabete Weiderpass,7 Corina S Rueegg,2 Marit B Veierød1 1Oslo Centre for Biostatistics and Epidemiology, Department of Biostatistics, Institute of Basic Medical Sciences, University of Oslo, Oslo, Norway; 2Oslo Centre for Biostatistics and Epidemiology, Oslo University Hospital, Oslo, Norway; 3Cancer Registry of Norway, Institute of Population-Based Cancer Research, Oslo, Norway; 4QIMR Berghofer Medical Research Institute, Brisbane, QLD, Australia; 5Cancer Research UK Manchester and Faculty of Biology, Medicine and Health, University of Manchester, Manchester, UK; 6Department of Community Medicine, Faculty of Health Sciences, UiT The Arctic University of Norway, Tromsø, Norway; 7International Agency for Research on Cancer, Lyon, FranceCorrespondence: Flavie Perrier, Oslo Centre for Biostatistics and Epidemiology, Department of Biostatistics, Institute of Basic Medical Sciences, University of Oslo, PO Box 1122, Blindern, Oslo, N-0317, Norway, Email flavie.perrier@medisin.uio.noPurpose: Physical activity (PA) is a cornerstone in disease prevention and varies throughout life. A pooled analysis of cohort studies and a meta-analysis of cohort studies found positive associations between PA and melanoma risk. However, previous studies focused on PA at specific ages and often lacked information on ultraviolet radiation (UVR) exposure. Using the population-based Norwegian Women and Cancer (NOWAC) cohort, including information on PA and UVR exposure, we estimated life-course PA trajectories from adolescence to adulthood and their associations with melanoma.Methods: Total PA across different domains (recreation, occupation, transport, household) was reported for ages 14 and 30 years, and when responding to the questionnaire (31– 76 years) using a 10-point scale, validated to rank PA levels in Norwegian females. We estimated life-course PA trajectories using a latent class mixed model in 152,248 women divided into three subcohorts depending on age at questionnaire completion: 31– 39 (n = 27,098), 40– 49 (n = 52,515) and ≥ 50 years (n = 72,635). The unique 11-digit identity number of Norwegian citizens was used to link NOWAC to the Cancer Registry of Norway for information on cancer diagnoses, emigration and death. Associations between PA trajectories and melanoma risk were estimated in each subcohort using multivariable Cox regression.Results: Five classes of individual life-course PA trajectories were identified in subcohort 31– 39 years (low, moderate, high, decreasing, increasing PA) and four in subcohorts 40– 49 and ≥ 50 years (low, moderate, high, decreasing PA). No significant association was found between life-course PA trajectories and melanoma risk in any subcohort. Hazard ratios (95% confidence intervals) for the high versus moderate trajectory were 0.92 (0.66– 1.29), 1.15 (0.97– 1.37) and 0.90 (0.78– 1.05) for subcohorts 31– 39, 40– 49 and ≥ 50 years, respectively.Conclusion: Our results do not support a positive association between PA and melanoma risk found in previous studies, which is important for public health guidelines promoting regular PA.Keywords: physical activity, cutaneous melanoma, trajectory, cohort, women, Norway