STUDY DESIGN: Fifty-two practices in the East Midlands, United Kingdom, were included. OBJECTIVES: To test the hypothesis that referral for lumbar spine radiography is cost-effective in primary care patients with low back pain of at least 6 weeks' duration compared with usual care in which referral is not routine. SUMMARY OF BACKGROUND DATA: Lumbar spine radiography is commonly used in the management of low back pain, although the yield of findings that alter clinical management is low. Evidence is needed on the cost-effectiveness of lumbar spine radiographs in patients with low back pain. METHODS: A prospective economic analysis alongside a randomized controlled trial was used. Outcomes included the Roland disability score, pain, health status scale, EuroQol, satisfaction, direct health care costs (primary, secondary, and community care; prescribed and over-the-counter medicines; special equipment), and indirect costs (informal care, extra expenses, welfare benefits, loss of earnings and productivity). RESULTS A total of 210 participants were randomly assigned to lumbar spine radiography, and 211, to usual care. At 9 months' postrandomization, no difference between the groups was found in any health outcomes other than satisfaction. The intervention group had a higher overall satisfaction score (21 19, < 0.01). The intervention group had higher direct costs (150 pounds sterling vs 109 pounds sterling, < 0.01). Cost-effectiveness analysis shows that patient satisfaction can be increased using lumbar radiography but at an additional cost (point estimate 20 pounds sterling per point on satisfaction scale). The simulated distribution based on trial data shows that only when a 1-point increase in satisfaction is valued at more than 50 pounds sterling can it be claimed that radiography is cost-effective in these terms (incremental net monetary benefit mean = 116 pounds sterling, 95% CI pound 7, 225 pounds sterling). CONCLUSIONS: Radiography is likely to be cost-effective only when satisfaction is valued relatively highly. Strategies to enhance satisfaction for patients with low back pain without using lumbar radiography should be pursued.