Pilot studies and a literature review suggested that fear-avoidance beliefs about physical activity and work might form specific cognitions intervening between low back pain and disability. A Fear-Avoidance Beliefs Questionnaire (FABQ) was developed, based on theories of fear and avoidance behaviour and focussed specifically on patients' beliefs about how physical activity and work affected their low back pain. Test-retest reproducibility in 26 patients was high. Principal-components analysis of the questionnaire in 210 patients identified 2 factors: fear-avoidance beliefs about work and fear-avoidance beliefs about physical activity with internal consistency (alpha) of 0.88 and 0.77 and accounting for 43.7% and 16.5% of the total variance, respectively. Regression analysis in 184 patients showed that fear-avoidance beliefs about work accounted for 23% of the variance of disability in activities of daily living and 26% of the variance of work loss, even after allowing for severity of pain; fear-avoidance beliefs about physical activity explained an additional 9% of the variance of disability. These results confirm the importance of fear-avoidance beliefs and demonstrate that specific fear-avoidance beliefs about work are strongly related to work loss due to low back pain. These findings are incorporated into a biopsychosocial model of the cognitive, affective and behavioural influences in low back pain and disability. It is recommended that fear-avoidance beliefs should be considered in the medical management of low back pain and disability.