OBJECTIVE To investigate the relationships among chronic low-back pain and obesity, total spinal range of motion, and trunk muscle strength. The short-term impact of trunk muscle strengthening exercises on this condition was also examined. METHODS A controlled, prospective study of trunk muscle strengths of patients with chronic low-back pain and the short-term impact of exercise on strength. The study group consisted of 25 female patients who had been experiencing low-back pain for at least 3 mo, and the control group included 20 age-matched women without known low-back trouble. The Davenport Index was used to calculate the body mass indexes of all subjects. The Oswestry Disability Questionnaire was used to assess pain in the study group. Full flexion and extension ranges of motion were measured, then isokinetic measurements of trunk muscles were performed at 60-, 120-, and 180-degrees/sec velocities. Isometric measurements were also recorded for both flexors and extensors at a 60-degree angle. RESULTS Increased body mass index and decreased trunk muscle strength were found to be directly associated with chronic low-back pain (P < 0.05). After a 15-day standard trunk strengthening exercise program in the patient group, trunk muscle strength was found to be increased (P < 0.05). CONCLUSIONS Obesity and decrease in trunk muscle strength are important factors in chronic low-back pain, and a trunk muscle strengthening program will be helpful in reducing the pain.