To encourage the clinical use of functional status measures, tools have been developed that are shorter and easier to administer. This study was conducted to provide empirical evidence concerning the validity of instrumental activities of daily living (IADL) scales. Similar to previous findings, this study found that a 50-item questionnaire was suitable for patient self-completion. This scale also showed good reproducibility over a 2-week period, a unidimensional factor structure, internal consistency, predictable correlations with other health status measures, and the ability to distinguish formal service users from nonusers. On the other hand, the findings also clearly demonstrate that caution is warranted when choosing rating formats and scoring procedures, and when interpreting patient responses. Ratings of difficulty, nonperformance, and assistance received should not be assumed to be interchangeable nor to reflect underlying physical ability. A great deal of sample variability was found when the process validity, or the subjective meanings of these ratings was examined. Scalogram analysis also indicated that the assumption of hierarchical ordering is not justified. Based on these findings, minimal questioning, dichotomous responses categories, and aggregate scores are not recommended for discharge planning or monitoring individual patients.