Background: Diagnosing dehydration in frail older persons is challenging. Objective: In residents of long-term geriatric and palliative care to appraise which clinical signs and laboratory data are associated with dehydration. Methods: Study Part I is a cross-sectional point of care assessment of data which might distinguish dehydrated from euhydrated subjects. Twelve potential markers of dehydration were evaluated: inadequate fluid intake, vomiting, diarrhea, bleeding, diuretic treatment, serum sodium, serum urea and creatinine, urea/creatinine ratio, estimated glomerular filtration rate, hemoglobin and serum albumin. Study Part II is a longitudinal survey of patients at risk of dehydration under changing clinical conditions. He clinical and laboratory data were prospectively followed and related to the patients' hydration state. Results: By point-of-care assessment (Study Part I) no single clinical or laboratory parameter correlated with dehydration. On longitudinal survey (Study Part II), useful in the diagnosing of dehydration were patient history corroborated with clinical and laboratory parameters designed 'potential markers of dehydration'. Seven case studies illustrate a variety of scenarios under which dehydration may occur. Conclusions: Diagnosing dehydration in residents of longterm geriatric and palliative care is challenging. Useful to this aim are the day-today examination of the patient by the same clinician, with or without the support of conventional 'laboratory markers of dehydration'. Overemphasis and dependence on laboratory markers may be mislead the physician.