OBJECTIVE: To determine the relationship between an audible pop and symptomatic improvement with spinal manipulation in patients with low back pain (LBP). DESIGN: A prospective cohort study. SETTING: Two outpatient physical therapy clinics located in military medical centers. PARTICIPANTS: A cohort of 71 patients with nonradicular LBP referred to physical therapy. INTERVENTIONS: Participants underwent a standardized examination and standardized spinal manipulation treatment program. All patients were treated with a sacroiliac (SI) region manipulative technique and the presence or absence of an audible pop was noted. Main Outcome Measures: Subjects were reassessed 48 hours after the manipulation for changes in range of motion (ROM), numeric pain rating scale (PRS) scores, and modified Oswestry Disability Questionnaire (ODQ) scores. RESULTS: An audible pop occurred in 50 of the 71 subjects during the manipulative procedure. Both groups-those who had an audible pop and those who did not-improved over time in flexion ROM, PRS scores, and modified ODQ scores; however, there were no differences between groups (P>.05). Nineteen of the 71 (27%) patients improved dramatically (mean drop in modified ODQ, 67.6%). In 14 of the 19 dramatic responders, an audible pop occurred. However, the odds ratio (1.2; 95% confidence interval, 0.38-4.04) suggested that the occurrence of a manipulative pop would not improve the odds of achieving a dramatic reduction in symptoms after the manipulation. CONCLUSION: There is no relationship between an audible pop during SI region manipulation and improvement in ROM, pain, or disability in individuals with nonradicular LBP. Additionally, the occurrence of a pop did not improve the odds of a dramatic improvement with manipulation treatment.