The radiculographic and computed tomographic findings in those nerve roots compressed by lumbar lateral canal entrapment have been assessed in a prospective series of 44 patients in whom such entrapment was found at surgery. The radiological procedures were reported by radiologists who were unaware of the clinical findings. Radiculography was performed in 41 of the patients and enabled the site or sites of entrapment to be correctly predicted in 62% of patients undergoing primary decompression and in 42% of patients undergoing revision surgery. Computed tomography (CT) was performed in 24 of the patients and enabled entrapment to be correctly predicted in 75% of the patients in both groups. We conclude that computed tomography is superior to radiculography in demonstrating lumbar lateral entrapment, and should usually be reserved for patients in whom operation or other intervention is warranted on clinical grounds. Otherwise false positive results may occur, especially in elderly patients, although the true incidence of false positive results in this condition is not known and has not been assessed in this study.