Animal experiments and studies in humans clearly show that the relation between pain (acute and chronic) and sleep quality is two-way: sleep disorders can increase pain, which in turn may cause sleep disorders. Sleep disorders and chronic low back pain are frequent health problems and it is unsurprising that the two can co-exist. This study was conducted to evaluate if sleep disorders and chronic pain associated are more frequently than one would expect. The objective of the study was to compare sleep quality in a population of patients with chronic low back pain and a control population. Sleep quality was assessed in 101 patients with chronic low back pain (CLBP) and in 97 sex- and age-matched healthy control subjects using the Pittsburgh Sleep Quality Index [PSQI; score from 0 (no disorder) to 21]. The French version of the Dallas Pain Questionnaire (DPQ) was used to assess the impact of low back pain on patients' quality of life. This impact was taken as nil in the healthy controls. The patients with CLBP and the controls were comparable in age, sex, and height, but mean bodyweight was higher in the CLBP group (70.3 +/- 14.5 vs. 61.8 +/- 11.4 kg; P < 0.05). The patients with CLBP were also more frequently on sick leave than the controls (32.3%; n = 31 vs. 0.0% n = 0; P < 0.001). Coffee, tea, and cola intakes were comparable in the two groups. Patients with CLBP had statistically higher scores in all items of the PSQI than the healthy controls. The mean PSQI was 4.7 +/- 3.2 for the healthy controls and 10.9 +/- 7.9 for the patients with CLBP (P < 0.0001). Sleep disorders were greater when the impact of CLBP on daily life (the four aspects of the DPQ) was greater [P < 0.0001]). The sleep of the patients with CLBP was significantly altered compared with that of the healthy controls, in proportion to the impact of low back pain on daily life. Our findings do not indicate whether sleep disorders are a cause or a consequence of CLBP.