Abstract Background Atrial fibrillation (AF) is associated with increased mortality, cardiovascular morbidity, and risk of stroke. With aging population, the number of AF patients is increasing, and they present with more comorbidities representing a major public health challenge. Purpose The influence of concomitant diseases on survival of patients with AF is not well established. Using data from a comprehensive nationwide cohort study, we studied the influence of several comorbidities on one-year mortality after the diagnosis of new-onset AF. Methods All patients with newly diagnosed AF in Finland 2007-17 were identified from nationwide registries. Individualized data on concomitant diseases were gathered from ICD-10 diagnoses during hospitalizations and outpatient visits in primary and specialist care and from drug reimbursements registries. Patients were followed until the end of 2018 from the causes of death registry, which records every death in the country. All-cause mortality associated with several comorbidities per 1000 person-years was determined, and relative risk (RR) of death was calculated. Results A total of 229,321 patients (mean age 73 ± 13.2 years, 50.0% male) with new-onset AF were identified. Mean age of men and women with new-onset AF was 69.4 ± 13.4 and 77.1 ± 11.8 years, respectively. During the one-year follow-up, 11.6% of men and 13.7% of women died. Heart failure, kidney disease and dementia were the strongest predictors of mortality and were associated with an over threefold risk of death within one year. Cancer, history of bleeding and vascular disease carried over twofold risk of death. In addition, hypertension, diabetes, alcohol abuse and history of stroke predicted mortality. The risk of death associated with these comorbidities is presented in Table. Conclusions Several comorbidities were associated with significantly increased risk of death within one year of the AF diagnosis. Heart failure, kidney disease and dementia were strongest predictors of subsequent mortality with over threefold risk. These data will inform the treatment decisions regarding AF patients with comorbidities.