Undernutrition is a common problem associated with clinical complications such as impaired immune response, reduced muscle strength, impaired wound healing, and susceptibility to infections; therefore, it is an important treatment target to reduce morbidity and mortality associated with chronic diseases and aging. The aim of the present study was to apply a reliable and valid instrument for the determination of undernutrition risk in an in-hospital patient population and to describe possible associations between risk of undernutrition and some aspects of health-related quality of life in patients with chronic diseases. Fifty-six adult patients with different chronic diseases were interviewed with NUFFE questionnaire and the EQ-5D. Anthropometric measurements were performed. Reliability and validity of the NUFFE instrument was tested, and its correlation with EQ-5D was calculated. Euro-Qol scores correlated significantly with the total NUFFE scores and with the items constructing the most important factor of the instrument, explaining 53.74% of its variance. Nutritional form for the elderly was shown to be a reliable instrument in the study group because its internal consistency measured by Cronbach alpha was 0.62, and the item-total score correlations were significant for the half of the items. Criterion-related validity, concurrent validity, and construct validity of NUFFE were established. We have shown that impaired level of health-related quality of life is an important determinant of risk for undernutrition. Nutritional form for the elderly is an appropriate instrument to estimate undernutrition risk in a general, in-hospital patient population with various chronic diseases and to identify "at risk" patients who may benefit from professional dietary interventions to reduce undernutrition-related complications.