Background: Patients with under-treated asthma i.e. uncontrolled asthma in treatment step 1-3, remain poorly studied. Aim: To study factors associated with under-treated asthma in a large adult asthma cohort. Methods: We studied 28584 asthma patients (?18yr) registered in the Swedish National Airway Register between 2017 and 2019. We linked the recordings to other national registers to obtain information on prescribed drugs up to a 2-year pre-index period, and education. Patients were classified into treatment step 1-3 and 4-5, and patients in step 1-3 with uncontrolled asthma were further defined as under-treated asthma since treatment escalation is possible. Uncontrolled asthma was defined as: ACT ?19 and/or previous exacerbations and/or FEV1<80% predicted. Future exacerbations (oral corticosteroids; OCS, or hospitalization) were identified in a 1-year post-index period. Results: Step 1-3 included; 17318 patients, of which 9586 (55%) were under-treated, and step 4-5; 11266 patients. In adjusted analyses, female sex, older age, overweight, obesity, current smoker, primary education and secondary education were associated with under-treated asthma. Under-treated asthma had similar OR as step 4-5 treatment when analyzing associations with future exacerbations: OCS?2 (under-treated: ORadj 2.11, 95% CI 1.94-2.30; step 4-5: 2.55, 2.35-2.77), and hospitalization (under-treated: 2.66, 2.26-3.12; step 4-5: 3.79, 3.25-4.41). Conclusions: Under-treated asthma in step 1-3 is common, and associates with female sex, current smokers, higher BMI, and lower socio-economic status. After evaluation of inhaler technique and adherence, appropriate escalation of treatment is important as these patients have an increased risk for future exacerbations.