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Background Estimates of long-term atrial fibrillation (AF) incidence in individuals with sinus rhythm are limited. This study investigated age-stratified and sex-stratified long-term incidence of AF in individuals with baseline sinus rhythm and no history of AF, and examined associations between AF, ischaemic stroke and overall cardiovascular disease (CVD).Methods Participants were recruited to the population-based Interplay Between Genetic Susceptibility and External Factors cohort, initiated in western Sweden (2001–2004). Follow-up of incident diagnoses and mortality continued through national registers until 31 December 2022. Cumulative incidence of AF was calculated accounting for the competing risk of death. Cox regression models assessed associations between AF as a time-varying exposure, incident ischaemic stroke and overall CVD, adjusting for confounding.Results 2967 AF-free participants were included (mean age 50.7±13.4 years; 52.9% female). During 53 447 person-years of follow-up (median 20.1 years), 356 (12.0%) developed AF (incidence rate 6.66 per 1000 person-years). At 20 years, the cumulative incidence of AF accounting for competing risk of death was 2.1% (<45 years at baseline), 13.3% (45–65 years) and 29.5% (>65 years). Men had a higher incidence than women in all age groups. Incident AF was associated with ischaemic stroke (HR 1.80, 95% CI 1.19 to 2.72) and overall CVD (HR 2.07, 95% CI 1.60 to 2.68).Conclusions AF was a common long-term outcome, particularly among older adults and men, and was associated with subsequent stroke and CVD. Age-specific and sex-specific risk stratification may inform targeted follow-up and early AF detection.