In a randomized controlled trial plus a nonrandomized cohort, the authors investigated the effectiveness and costs of acupuncture in addition to routine care in the treatment of chronic low back pain and assessed whether the effects of acupuncture differed in randomized and nonrandomized patients. In 2001, German patients with chronic low back pain were allocated to an acupuncture group or a no-acupuncture control group. Persons who did not consent to randomization were included in a nonrandomized acupuncture group. All patients were allowed to receive routine medical care in addition to study treatment. Back function (Hannover Functional Ability Questionnaire), pain, and quality of life were assessed at baseline and after 3 and 6 months, and cost-effectiveness was analyzed. Of 11,630 patients (mean age=52.9 years (standard deviation, 13.7); 59% female), 1,549 were randomized to the acupuncture group and 1,544 to the control group; 8,537 were included in the nonrandomized acupuncture group. At 3 months, back function improved by 12.1 (standard error (SE), 0.4) to 74.5 (SE, 0.4) points in the acupuncture group and by 2.7 (SE, 0.4) to 65.1 (SE, 0.4) points among controls (difference=9.4 points (95% confidence interval 8.3, 10.5); p<0.001). Nonrandomized patients had more severe symptoms at baseline and showed improvements in back function similar to those seen in randomized patients. The incremental cost-effectiveness ratio was euro10,526 (euros) per quality-adjusted life year. Acupuncture plus routine care was associated with marked clinical improvements in these patients and was relatively cost-effective.