Most studies investigating the course of recent-onset low back pain (LBP) included patients from primary care. We aimed to describe the prognosis in people with recent-onset LBP presenting to emergency departments (EDs) and to identify prognostic factors for non-recovery. This inception cohort study with a one-year follow-up recruited 600 consecutive acute LBP patients presenting to four EDs. The outcomes measured the days to recover from pain, recover from disability, return to previous work hours and duties, and complete recovery. Within 12 months, 73% of participants (95% CI=69 to 77) recovered from pain, 86% (95% CI=82 to 90) recovered from disability, 75% (95% CI=67 to 82) returned to previous work hours and duties, and 68% (95% CI=64 to 72) completely recovered. The median recovery times were 67 days (95% CI=54 to 80) to recover from pain, 37 days (95% CI=31 to 43) to recover from disability, 37 days (95% CI=25 to 49) to return to previous work hours and duties, and 71 days (95% CI=56 to 85) to recover completely. Higher pain levels, a higher perceived risk of persistent LBP, more days of reduced activity due to LBP, more pain sites, and higher duration of LBP were associated with complete non-recovery within six months. Perspective: This information relates to prognosis and to likely recovery times for patients with recent-onset LBP in EDs. The findings also confirm previous factors associated with poor outcomes in patients with recent-onset LBP.