['To investigate the prevalence and associated factors of severe low back pain (LBP) among patients with rheumatoid arthritis (RA).', 'This cross-sectional study included 201 patients with RA without prior spinal surgery. Severe LBP was defined as that with a visual analog scale (VAS) score of ≥ 50 mm within the previous 4 weeks. Lumbar lesions, sagittal alignment, and disc degeneration were evaluated by plain standing X-rays and magnetic resonance imaging. Associated factors of severe LBP were evaluated using multiple logistic regression analysis.', 'Forty-eight patients (23.8%) had LBP with a VAS score of ≥ 50 mm. Multivariate analysis indicated that the associated factors for severe LBP were female, smoking, and moderate and high disease activity on the Disease Activity Score in 28 joints-erythrocyte sedimentation rate (DAS28-ESR). There was no relationship between severe LBP and any radiological findings. Among DAS28-ESR subscores, patients with severe LBP had significantly higher tender joint counts and VAS scores for general health.'] Is severe low back pain in patients with rheumatoid arthritis associated with Disease Activity Score but not with radiological findings on plain X-rays?