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This study was carried out to determine the dose-response relationship of the effects of prostaglandin E1 (PGE1) on the nerve root blood flow and the neurologic symptoms after lumbar decompressive surgery using laser Doppler flowmetry.Fifty-four patients undergoing lumbar laminectomy were randomly assigned to one of three groups. Each group received intravenously one of three test solutions; 10 ml of saline in group A (N = 18), 10 ml of saline containing PGE1, 20 μg, in group B (N = 18), and 10 ml of saline containing PGE1, 50 μg, in group C (N = 18). After lumbar laminectomy, a probe of laser Doppler flowmetry was placed directly on the lumbar nerve root. Nerve root blood flow (RBF) velocity was measured before the injection of test solution (T1), at 5 min (T2) and 10 min (T3) after starting injection, and 10 min after the end of injection (T4). A visual analogue scale (VAS) and the walking distance were measured for assessment of neurologic symptoms before and after surgery.In group A, RBF velocity showed no change throughout the time course. The RBF velocity in groups B and C showed a significant increase at T2 (120% in group B and 122% in group C, p < 0.05), T3 (124% in group B and 127% in group C, p < 0.05) and T4 (132% in group B and 131% in group C, p < 0.01) compared with T1. The scores of VAS in groups B and C were significantly (p < 0.05) lower than those in group A and the walking distances in groups B and C were significantly (p < 0.05) longer than those in group A at 5th postoperative day.We conclude that low and moderate doses of PGE1 cause an increase in nerve root blood flow after lumbar decompressive surgery, and show an improvement of neurologic symptoms in postoperative conditions. However, there seems to be a ceiling effect of PGE1 at the dose of 20 μg or less.