BACKGROUND: Lumbar spine radiography has limited use in diagnosing the cause of acute low back pain. Consensus-based guidelines recommend that lumbar spine x-rays are not used routinely. However there have been no studies of the effect of referral for radiography at first presentation with low back pain in primary care. AIM: To compare short and long-term physical, social, and psychiatric outcomes for patients with low back pain who are referred or not referred for lumbar spine x-ray at first presentation in general practice. DESIGN OF STUDY: A randomised unblinded controlled trial with an observational arm to enable comparisons to be made with patients not recruited to the trial. SETTING: Ninety-four general practices in south London and the South Thames region. METHOD: Patients consulting their general practitioner (GP) with low back pain at first presentation were recruited to a randomised controlled trial (RCT) or to an observational group. Patients in the trial were randomly allocated to immediate referral for x-ray or to no referral. All patients were asked to complete questionnaires initially, and then at six weeks and one year after recruitment. RESULTS: Six hundred and fifty-nine patients were recruited over 26 months: 153 to the randomised trial and 506 to the observational arm. In the RCT referral for x-ray had no effect on physical functioning, pain or disability, but was associated with a small improvement in psychological wellbeing at six weeks and one year. These findings were supported by the observational study in which there were no differences between the groups in physical outcomes after adjusting for length of episode at presentation; however, those referred for x-ray had lower depression scores. CONCLUSIONS: Referral for lumbar spine radiography for first presentation of low back pain in primary care is not associated with improved physical functioning, pain or disability. The possibility of minor psychological improvement should be balanced against the high radiation dose involved.