Undernutrition among elderly patients is a significant problem in nursing care. The aims of this study were to develop and test an instrument for identifying actual and potential undernutrition among elderly patients in clinical nursing care. A Likert-type scale consisting of 15 items was constructed. A consecutive sample of 56 elderly patients (>65 years) in a geriatric rehabilitation ward in western Sweden were interviewed with the instrument. The data were mainly analysed with non-parametric statistical methods. The results showed that the instrument was a fairly reliable scale with a Cronbach's alpha coefficient of 0.72. Evidence of validity concerning face validity, criterion-related validity—including concurrent and predictive validity—and construct validity was shown in the study group. Further testing is required if the instrument is to be used in clinical nursing care and research.