This study sought to provide further information about the psychometric properties of the Clinical Impression of Severity Index for Parkinson's Disease (CISI-PD), in a large, international, cross-culturally diverse sample. Six hundred and fourteen patients with PD participated in the study. Apart from the CISI-PD, assessments were based on Hoehn & Yahr (HY) staging, the Scales for Outcomes in PD-Motor (SCOPA-M), -Cognition (SCOPA-COG) and -Psychosocial (SCOPA-PS), the Cumulative Illness Rating Scale-Geriatrics, and the Hospital Anxiety and Depression Scale. The total CISI-PD score displayed no floor or ceiling effects. Internal consistency was 0.81, the test-retest intraclass correlation coefficient was 0.84, and item homogeneity was 0.52. Exploratory and confirmatory factor analysis (CFI = 0.99, RMSEA = 0.07) confirmed CISI-PD's unifactorial structure. The CISI-PD showed adequate convergent validity with SCOPA-COG and SCOPA-M (r(S) = 0.46-0.85, respectively) and discriminative validity for HY stages and disease duration (P < 0.0001). In a multiple regression model, main CISI-PD predictors were SCOPA-M, disease duration, and depression. The results obtained were not only comparable to but also extended those yielded by the preliminary validation study, thus showing that the CISI-PD is a valid instrument to measure clinical impression of severity in PD. Its simplicity and easy application make it an attractive and useful tool for clinical practice and research.