Substance P-like immunoreactivity (SP-LI) of the cerebrospinal fluid was measured by radioimmunoassay in 40 patients with lumbar disc herniation (hernia group), and in 10 patients with no low back pain and no leg symptoms (control group). The SP-LI was significantly higher in the hernia group (5.49 +/- 3.01 pg/ml) than in the control group (2.05 +/- 0.52 pg/ml) (p < 0.01). In the hernia group, the SP-LI was significantly higher in patients with severe pain in the lower extremities than in those with only mild pain. As the SP-LI was found to be correlated with the severity of pain, it was considered to be a useful index of pain. As for the correlation of SP-LI with the hernia type, the SP-LI was significantly higher in patients with transligamentous extrusion type hernia than in those with protrusion type hernia. This result suggested that the release of substance P was increased with marked compression on the dorsal root.