Objective Recent research suggests that sleep quality and pain intensity are intimately linked. Although sleep problems are common in patients with low back pain, the effect of sleep quality on the levels of pain intensity is currently unknown. The aim of this study was to investigate the effect of sleep quality on subsequent pain intensity in patients with recent-onset low back pain. Methods Data on 1,246 patients with acute low back pain were included in the analysis. Sleep quality was assessed using the sleep quality item of the Pittsburgh Sleep Quality Index, scored on a 0-3-point scale, where 0 = very good sleep quality and 3 = very bad sleep quality. Pain intensity was assessed on a numerical rating scale (range 0-10). A generalized estimating equation (GEE) analysis modeled with an exchangeable correlation structure was used to examine the relationship between sleep quality and pain intensity. The model further controlled for symptoms of depression and prognostic factors for low back pain. Results The GEE analysis demonstrated a large effect of poor sleep on subsequent pain intensity, such that for every 1-point decrease in sleep quality (based on a 0-3-point scale), pain intensity (based on a 0-10-point scale) increased by 2.08 points (95% confidence interval 1.99-2.16). This effect was independent of depression and common prognostic factors for low back pain. Conclusion Sleep quality is strongly related to subsequent pain intensity in patients with acute low back pain. Future research is needed to determine whether targeting sleep improvement contributes to pain reduction.