In a prospective, consecutive study of patients with lumbar back pain and sciatica, various clinical features and surgical findings were evaluated in order to analyse the predictive value regarding (1) level of diseased interspace (2) presence and type of lesion responsible for root compression (3) outcome after surgery. One hundred patients underwent surgery solely on clinical grounds. Fifty-eight had disc herniation. The level of disc herniation was correctly predicted in three quarters of patients with a prolapsed disc. The outcome after surgery was good in 77 patients. Only few clinical features, namely male sex and scoliosis were predictors of a good outcome. Lasegue's sign was indicative of root compression in 90%, but only two-thirds had disc herniation. Conversely one-third had disc herniation in spite of a "negative" test. Lasegue's sign was not superior to other clinical tests in predicting outcome. The most important indicator of a good outcome was the presence of disc herniation at surgery. Patients with disc pathology other than true disc herniation fared equally with patients, who had normal discs disclosed at surgery. Myelography was undertaken in all patients prior to surgery, the results of which are analysed in the following paper. (Espersen et al.: Predictive value of radiculography in patients with lumbago-sciatica. A prospective study (Part II, Acta Neurochirurgica 73 (1984), 213-221.