Abstract Background To improve the diagnostic performance of the cerebrospinal fluid (CSF) tap test (TT), early and delayed assessments of gait were performed after the removal of 30 ml of CSF in patients with probable idiopathic normal pressure hydrocephalus. Assessments of gait included the 3-m timed up and go test (TUG), and the 10-m walk in time (10Ti) and in step (10St) tests. Methods Quantitative data for the TUG, the 10Ti, and the 10St were obtained before CSF removal and on days 1 and 4 after removal of 30 ml CSF. CSF shunt surgery was performed in 61 patients within one month after the TT. The gait outcome was assessed 3 months after surgery. The area under the curve (AUC), sensitivity, specificity, and cutoff values were computed for the TUG, the 10Ti, and the 10St on day 1 and day 4 using receiver operating characteristic (ROC) curve analysis. Results The positive response rate in three measures on day 4 was equal to or higher than the values on day 1. Times were reduced significantly in the TUG and the 10mTi tests between baseline and both days 1 and 4 after TT. No significant differences were noted in the number of steps for the 10St test. The percent change in TUG on day 1 had the highest AUC value among all other variables (0.808). Although this was not statistically different from other variables in the TUG and the 10Ti, it had a good balance of high sensitivity (78.3%) and high specificity (80.0%), with a cutoff value of 11.3%. The change in the measured value in the day 1 TUG had the second highest AUC value at 0.770. The variables on day 4 tended to have high specificities of around 90%, although their sensitivities were low. Conclusions The percent change of TUG on day 1 showed the highest diagnostic accuracy. Delayed assessments on day 4 were not superior to those on day 1. Thus, the TUG on day 1 is useful as a simple quantitative measure for predicting shunt effectiveness.