To determine the diagnostic efficacy of late responses and of magnetic stimulation in the electrodiagnostic evaluation of lumbosacral radiculopathies, 42 patients with acute monoradiculopathies of L5 or S1 were examined. We performed conventional nerve conduction studies, F-wave studies, needle electrode examination (NEE) and magnetic stimulation. The results were compared with a control group of 36 persons. In the patients with weakness, we found a diagnostic sensitivity for NEE of 90% in L5 and of 80% in S1. F-waves had the same sensitivity as NEE in the patients with weakness and were more sensitive in the group of patients without weakness (L5 80%, S1 67%). Magnetic stimulation had a sensitivity of 40% in all groups. There were also abnormalities of NEE and of F-wave studies in the patients with abnormal magnetic stimulation. It is concluded that NEE is the single most effective method in acute LSR and that F-wave studies are able to provide complementary information. Magnetically evoked motor nerve root stimulation was not found of clinically relevant diagnostic value.