OBJECTIVE To investigate whether illness perceptions, measured with the Brief Illness Perception Questionnaire, are an independent predictor of chronic low back pain and pain-related disability at 12 weeks. METHODS A prospective, observational cohort study. METHODS 26 outpatient primary care physiotherapy practices throughout the Netherlands. METHODS Acute nonspecific low back pain patients between the age of 18 and 60 years, with or without radiating pain, and a pain-free episode of at least three months before onset. METHODS Standard physiotherapy care according to Dutch clinical practice guidelines. METHODS Chronic low back pain defined as pain ≥3/10 on the Numeric Pain Rating Scale and as pain-related disability ≥19/70 on the Pain Disability Index measured after 12 weeks. RESULTS Two hundred and four people with acute nonspecific low back pain completed both assessments. In the multivariable analyses, adjusted for pain intensity, disability, duration, radiating pain, depressed mood, associations of illness perceptions were OR 1.04 (95% CI: 1.01 to 1.08) for pain and 1.04 (95% CI: 0.99 to 1.09) for pain-related disability. CONCLUSIONS Illness perceptions independently predicted chronic low back pain but not pain-related disability at 12 weeks. The added predictive value of illness perceptions was relatively low.