Objective We previously developed normative data for a French version of the Hayling Sentence Completion Test (f-HSCT) for adults and elderly people. The present study aimed to evaluate the clinical utility of the f-HSCT norms in two clinical populations in which inhibition dysfunction has been largely documented, i.e., Parkinson's disease (PD) and schizophrenia. Method Eighty-five non-demented patients with idiopathic PD and 64 out-patients with schizophrenia completed the automatic and inhibition conditions of the f-HSCT. Time latencies and errors raw data of each patient were compared to the norms previously developed by the authors. Results In the automatic condition, errors were rare in both clinical groups and time latencies on this condition felt within the normative data range. Compared with the standardized norms, 46% of patients with PD and 61% of patients with schizophrenia had a deviant performance (i.e., borderline or deficit) for the inhibition error score. The proportion of patients with a deviant performance on the inhibition response time score was similar in both clinical samples (respectively, 25% and 23%). Finally, slightly more than half of patients with PD and more than two-thirds of patients with schizophrenia had a deviant performance on at least one of the f-HSCT inhibition measures. Conclusions Our results suggest that the f-HSCT has a strong potential for characterizing inhibition of prepotent responses in PD and schizophrenia. Furthermore, it requires only a short administration time so it may be ideal to detect response inhibition in clinical populations with cognitive fatigue.