In this prospective study, predictors of outcome were identified for patients (n = 116) who presented to their family physician with acute mechanical low back pain. Short-term outcome was measured by the number of days lost from work and longer term outcome was measured by disability at the six-week follow-up. Unlike other published work, this study did not find obesity or a history of previous back problems to be related to a poorer outcome from acute episodes of low back pain. Among those patients not involved in manual labor, a history of anxiety or depression was a significant predictor of both greater work loss and longer term disability. Among this same group, cigarette smoking was also found to be related to greater long-term disability from acute low back pain. Further study of this relationship is needed. The number of hours of manual labor performed daily was a strong predictor of poor outcome (both short- and long-term) of acute episodes of low back pain. Among both manual laborers and professional-technical workers, the number of days off work (at bed rest) prescribed by the physician was significantly related to greater absenteeism from work; the physician's diagnosis of an actual or possible disc problem was also related (P less than .05) to greater work loss among manual laborers. Neither of these factors, however, was related to longer term disability.