STUDY DESIGN: A prospective cohort study. OBJECTIVE: To study if low level of physical fitness was associated with increased low back pain (LBP) intensity at 30-month follow-up. SUMMARY OF BACKGROUND DATA: The evidence of low physical fitness as a risk factor for LBP is inconclusive due to contradictory results. METHODS: Study participants were 327 employees (women = 271, men = 56) at institutions for physically and mentally disabled persons. Physical fitness was measured by tests of: back extension and flexion endurance, flexibility and balance; and by self-assessed aerobic fitness, muscle strength, endurance, flexibility and balance, using visual analogue scales. Low back pain, lifestyle parameters, and physical and psychosocial work factors were assessed by questionnaires at baseline and at follow-up. Outcome was defined as an increase above 2 steps in average LBP intensity during the previous year (0-10). RESULTS: Persons with low level back endurance showed an insignificantly higher risk of increased LBP intensity (OR = 2.4, P = 0.076), whereas persons with medium level back endurance were at significantly higher risk (OR = 2.7, P = 0.034) compared with those with high level back endurance. The general association between isometric back extension endurance and increased LBP intensity was insignificant (P = 0.067). Persons with medium level self-assessed aerobic fitness were at lower risk of increased LBP intensity compared with those with high level (OR = 0.37, P = 0.02), although the general association of aerobic fitness was insignificant (0.066). Performance-based back flexion endurance, flexibility, and balance; and self-assessed muscle strength, endurance, flexibility, and balance were not associated with increased LBP intensity. CONCLUSION: The significant association between medium level back extension endurance and increased LBP intensity supports the finding of other studies that particularly back extension endurance is an important physical fitness component in preventing LBP and that the subcomponents of physical fitness are related in different ways to LBP.