The objective of this study was to compare patients' and physicians' expectations concerning the recovery time for acute back pain, and their determinants. A French national observational survey was performed between October and December 2005. Each physician was asked to include the first three patients aged between 20 and 70 years presenting with acute back pain (VAS > 40 mm) of less than 1 month's duration. A total number of 1982 patients, with a mean age of 48.4 +/- 11.8 years (52.2% men), were enrolled by 834 physicians. Patients and their physicians expected recovery to take the same amount of time in 60.3% of cases (Kappa = 0.43). In 17.4% of cases, patients predicted a shorter recovery time than physicians, and in 22.4% of cases, patients predicted a longer recovery time. Both patients and physicians expected recovery time to be longer in women, and in older, unmarried, obese, and non working patients. Expected recovery time was also longer in patients with no sports activities, low back pain (as opposed to pain at other sites), a high level of disability, a history of back pain, and no triggering factor. The intensity of pain and the number of days of sick leave taken did not affect the patient's prediction of recovery time, but did affect the physicians' expectations. Physicians considered professional status, analgesic intake and previous sick leave to be independent predictive factors, whereas patients did not. Expected recovery time, assessed shortly after the onset of acute back pain is influenced by most of the clinical and demographic factors usually considered risk factors for chronic low back pain. Patients' predictions of recovery time should be assessed in acute back pain, to identify risks for expected chronicity.