We made 10 measurements of Pourcelot's index of resistance (RI) in the anterior cerebral artery (RI-ACA) and basilar artery (RI-BA), and RI ratio (= RI-ACA/RI-BA) in seven cases of hydrocephalus before and after treatment. The mean values of RI-ACA (0.831 +/- 0.050, mean +/- SD), RI-BA (0.800 +/- 0.053) and RI ratio (1.039 +/- 0.030) before treatment were significantly higher than in normal infants. Two hours after treatment, the mean RI-ACA (0.654 +/- 0.099) was lower than in normal infants, and RI-BA (0.684 +/- 0.101) were normalized. Before treatment, the mean value of RI-ACA was higher than that of RI-BA. All RI ratios were higher than 1.00. After treatment, the mean RI-ACA was lower than the mean RI-BA. All RI ratios were normalized and lower than 1.00 (0.957 +/- 0.024).