['Follow-up study.', 'To estimate the influence of fear avoidance beliefs (FAB) on 1-year low back pain (LBP)-related sick leave.', 'As opposed to Anglo-Saxon and Northern European subjects, in Spanish LBP patients the influence of FAB on disability and quality of life is virtually irrelevant.', 'Twelve primary care and 9 hospital services from 7 different regions of Spain recruited 165 workers who visited the National Health Service for LBP. None was excluded. On their first visit and 14 days later, patients were given 2 independent Visual Analogue Scales (VAS) for LBP and leg pain, as well as the validated Spanish versions of the Fear Avoidance Beliefs (FABQ), Roland-Morris (RMQ), and SF-12 questionnaires. During the following 12-month period, the number of days on sick leave because of LBP was registered.', 'Differences in sick leave throughout the study period were associated with baseline differences in chronicity, severity of low back (not referred) pain, disability, FAB, physical and mental quality of life, being on sick leave when entering the study, and duration of previous sick leave. Ordinal logistic regression models showed that each additional point in the total FAB baseline score increases by 2.4% the odds of being on sick leave for up to 60 days during the following year and by 7.7% the odds of being sick listed for 61 days or more. Corresponding figures for FAB-Work scores are 4.2% and 11%. No variable modifies the effect of FAB on sick leave.'] Do fear avoidance beliefs influence duration of sick leave in Spanish low back pain patients?