Purpose: To assess psychometric properties of the Western Ontario and MacMasters Universities Osteoarthritis Index (WOMAC)-pf, an osteoarthritis (OA)-specific questionnaire used to establish the level of physical functioning in patients with late-onset sequels of poliomyelitis (LOSP) and Parkinson's disease (PD). Study design and setting: Unidimensionality (using principal component analyses [PCA] and Rasch analyses) was separately established for three diagnostic groups: OA (n = 288), LOSP (n = 168) and PD (n = 200). Additionally, differential item functioning (DIF) among the three diagnostic groups was assessed using ordinal regression (Polytomous Universal Model) analyses. Baseline data were used from an ongoing cohort study of these three patient populations. Results: Unidimensionality was adequate, with all items loading on the first principal component. The Rasch analyses revealed that item fit was generally good. Uniform and non-uniform DIF were found to be present among the three diagnostic groups in three and one of the 17 physical functioning subscale (WOMAC-pf) items, respectively. Conclusion: The WOMAC-pf is a unidimensional measure of physical functioning in patients with LOSP and PD, in addition to its established use in OA. © 2012 Informa UK, Ltd.