Study Design. A systematic review of randomized controlled trials. Summary of Background Data. The treatment of chronic low back pain is not primarily focused on removing an underlying organic disease but at the reduction of disability through the modification of environmental contingencies and cognitive processes. Behavioral interventions are commonly used in the treatment of chronic (disabling) low back pain. Objectives. To determine whether behavioral therapy is more effective than reference treatments for chronic nonspecific low back pain and which type of behavioral treatment is most effective. Methods. The authors searched the Medline and PsychLit databases and the Cochrane Controlled Trials Register up to April 1999, and Embase up to September 1999. Also screened were references of identified randomized trials and relevant systematic reviews. Methodologic quality assessment and data extraction were performed independently by two reviewers. The magnitude of effect was assessed by computing a pooled effect size for each domain (i.e., behavioral outcomes, overall improvement, back pain-specific and generic functional status, return to work, and pain intensity) using the random effects model. Results. Only six (25%) studies were high quality. There is strong evidence (level 1) that behavioral treatment has a moderate positive effect on pain intensity (pooled effect size 0.62; 95% confidence interval [Cl] 0.25, 0.98), and small positive effects on generic functional status (pooled effect size 0.35; 95% Cl: -0.04, 0.74) and behavioral outcomes (pooled effect size 0.40; 95% Cl: 0.10, 0.70) of patients with chronic low back pain when compared with waiting-list controls or no treatment. There is moderate evidence (level 2) that a addition of behavioral component to a usual treatment program for chronic low backpain has no positive short-term effect on generic functional status (pooled effect size 0.31; 95% Cl: -0.01, 0.64), pain intensity (pooled effect size 0.03; 95% Cl: -0.30, 0.36), and behavioral outcomes (pooled effect size 0.19; 95% Cl: -0.08, 0.45). Conclusions. Behavioral treatment seems to be an effective treatment for patients with chronic low back pain, but it is still unknown what type of patients benefit most from what type of behavioral treatment.