The spinal nerve injection study (SNIS) is a diagnostic test for the evaluation of sciatica. SNIS consists of selective infiltration of nerve root with local Xylocaine anesthesia. The most common indication is in the postlaminectomy patient with persistent or recurrent sciatica and peridural scarring causing myelograms or CT scans to be difficult to interpret. Of 105 patients with SNIS, 55 subsequently had surgical exploration of the suspected lesion. In patients with a diagnostic or highly suggestive SNIS, an accurate diagnosis was made in 43 (93%); myelograms in the same group had an accuracy of 24%. At follow-up evaluations ranging from 12 to 60 months (average, 20 months), 40 (73%) of the patients were improved by further surgical treatment.