Population-based, prospective cohort. To estimate incidence and course of severity-graded low back pain (LBP) episodes in adults. Past studies report variable estimates that do not differentiate LBP severity. An incidence cohort of 318 subjects free of LBP and a course cohort of 792 prevalent cases was formed from respondents to a mailed survey. Incident, recurrent, persistent, aggravated, improved, and resolved episodes were defined by the Chronic Pain Questionnaire. The follow-up at 6 and 12 months was 74% and 62%, respectfully. Annual estimates were age and sex standardized. The cumulative incidence was 18.6% (95% confidence interval [CI], 14.2%-23.0%). Most LBP episodes were mild. Only 1.0% (95% CI, 0.0%-2.2%) developed intense and 0.4% (95% CI, 0.0%-1.0%) developed disabling LBP. Resolution occurred in 26.8% (95% CI, 23.7%-30.0%), and 40.2% (95% CI, 36.7%-43.8%) of episodes persisted. The severity of LBP increased for 14.2% (95% CI, 11.5%-16.8%) and improved for 36.1% (95% CI, 29.7%-42.2%). Of those that recovered, 28.7% (95% CI, 21.2%-36.2%)had a recurrence within 6months,and 82.4% of it was mild LBP. Younger subjects were less likely to have persistent LBP (incidence rate ratio, 0.88; 95% CI, 0.80-0.97) and more likely to have resolution (incidence rate ratio, 1.26; 95% CI, 1.02-1.56). Most new and recurrent LBP episodes are mild. Less than one third of cases resolve annually, and more than 20% recur within 6 months. LBP episodes are more recurrent and persistent in older adults.