© 2015 Elsevier Inc. All rights reserved. Background context Primary care clinicians need to identify candidates for early interventions to prevent patients with acute pain from developing chronic pain. Purpose We conducted a 2-year prospective cohort study of risk factors for the progression to chronic pain and developed and internally validated a clinical decision rule (CDR) that stratifies patients into low-, medium-, and high-risk groups for chronic pain. Study design/Setting This is a prospective cohort study in primary care. Patient sample Patients with acute low back pain (LBP, ≤30 days duration) were included. Outcome measures Outcome measures were self-reported perceived nonrecovery and chronic pain. Methods Patients were surveyed at baseline, 6 months, and 2 years. We conducted bivariate and multivariate regression analyses of demographic, clinical, and psychosocial variables for chronic pain outcomes, developed a CDR, and assessed its performance by calculating the bootstrapped areas under the receiver-operating characteristic curve (AUC) and likelihood ratios. Results Six hundred five patients enrolled: 13% had chronic pain at 6 months and 19% at 2 years. An eight-item CDR was most parsimonious for classifying patients into three risk levels. Bootstrapped AUC was 0.76 (0.70-0.82) for the 6-month CDR. Each 10-point score increase (60-point range) was associated with an odds ratio of 11.1 (10.8-11.4) for developing chronic pain. Using a less than 5% probability of chronic pain as the cutoff for low risk and a greater than 40% probability for high risk, likelihood ratios were 0.26 (0.14-0.48) and 4.4 (3.0-6.3) for these groups, respectively. Conclusions A CDR was developed that may help primary care clinicians classify patients with strictly defined acute LBP into low-, moderate-, and high-risk groups for developing chronic pain and performed acceptably in 1,000 bootstrapped replications. Validation in a separate sample is needed.