The change in the relative size of lumbar disc hernias and its relation to sciatic symptoms was investigated in 30 consecutive patients after conservative treatment of CT verified lumbar disc herniations. CT and clinical examination were performed before the start of therapy (CT1), as well as 3 months (CT2) and 24 months (CT3) after institution of treatment. In each patient the size of the lumbar disc herniation in relation to the size of the spinal canal was measured on identical CT slices and expressed as an index. The disc herniation index decreased markedly from CT1 to CT2 (p less than 0.001). Between CT2 and CT3 the reduction of the hernias was less pronounced and not significant for hernias located centrally but still significant for intermediate (p = 0.03) and lateral (p = 0.04) hernias. The degree of sciatic symptoms also decreased markedly between CT1 and CT2 (p = 0.001) while no further improvement occurred from CT2 to CT3. There was a significant positive correlation between the improvement from sciatic pain and the reduction in the size of the individual hernia (CT1-CT2 p = 0.02, CT2-CT3 p less than 0.001). Thus, the disc herniation index provided a method to study the anatomic effect of conservative treatment as well as a method to evaluate sciatic symptoms in relation to anatomic changes.