The "Interaction with Disabled Persons" scale (IDP) has been used in research into baseline attitudes and to evaluate whether a shift in attitudes towards people with developmental disabilities has occurred following some form of intervention. This research has been conducted on the assumption that the IDP measures attitudes as a multidimensional construct and has good internal consistency. Such assumptions about the IDP appear flawed, particularly in light of failures to replicate its underlying factor structure. The aim of this study was to evaluate the construct validity and dimensionality of the IDP. This study used a prospective survey approach. Participants were recruited from first and second year undergraduate university students enrolled in health sciences, occupational therapy, physiotherapy, community and emergency health, nursing, and combined degrees of nursing and midwifery, and health sciences and social work at a large Australian university (n = 373). Students completed the IDP, a 20-item self-report scale of attitudes towards people with disabilities. The IDP data were analysed using a combination of factor analysis (Classical Test Theory approach) and Rasch analysis (Item Response Theory approach). The results indicated that the original IDP 6-factor solution was not supported. Instead, one factor consisting of five IDP items (9, 11, 12, 17, and 18) labelled Discomfort met the four criteria for empirical validation of test quality: interval level scaling (scalability), unidimensionality, lacked of DIF across the two participant groups and data collection occasions, and hierarchical ordering. Researchers should consider using the Discomfort subscale of the IDP in future attitude research since it exhibits sound measurement properties. (Contains 2 figures and 2 tables.)