Pelvic incidence was described as a fundamental parameter to describe spino-pelvic balance. In high-grade spondylolisthesis, severe dystrophic changes of the upper sacral endplate may be responsible for technical difficulties in pelvic incidence measurement. We propose to evaluate the reliability of PI measurement in high-grade spondylolisthesis patients and to compare the manual method with a computer-assisted method. In 30 high-grade spondylolisthesis patients, pelvic incidence was measured by manual and computer-assisted technique by the Spineview software package. We statistically assessed agreement between the manual and the computer-assisted technique, the intra-observer and the inter-observer reliability of the computer-assisted technique. Significant correlation was found (Spearman's rank R = 0.921 with P<0.001) between manual and computer-assisted results. The paired t test (t = 0.979 with P<0.001) and the intraclass correlation coefficient (ICC) were also significant. Intra- and inter-observer reliability of the computer-assisted technique were excellent with Spearman's rank correlation from 0.964 to 0.985 with P<0.001, a paired t test from 0.978 to 0.983 with P<0.001) and an ICC from 0.986 to 0.992. Intra- and inter-observer repeatability were better with the computer-assisted method than with the manual technique. We proved the reliability and repetability of a computer-assisted angular measurement method in high-grade spondylolisthesis patients. This validated measurement technique could be now used to measure the main parameters of the sagittal balance of the spine in further studies on spondylolisthesis patients.