β-endorphin-like immunoreactivity (iβ-EP) was measured in the CSF at myelography of 24 patients suspected of vertebral disk disease. Patients made several ratings of mood and pain for the 24 hours preceding myelography. Composite scores for pain, negative mood, and positive mood were derived by factor analysis. Pain Factor scores were negatively correlated with iβ-EP (r = −0.59, p < 0.001), indicating a possible role for iβ-EP in the perception of the severity of pain. No significant correlation was shown between Positive or Negative Mood Factor scores and CSF iβ-EP. A physiologic indicator of pain severity is discussed.