BACKGROUND: Dehydration occurs frequently in older patients and constitutes a significant clinical problem. OBJECTIVE: This proof-of-concept study examines whether 1) sublingual measurement in dehydrated old patients is feasible, 2) frailty and incompliance in old, awake patients affects video-quality, 3) dehydration impacts microcirculation METHODS: This prospective observational study included clinically dehydrated patients aged ≥65 years immediately after admission. Dehydration was assessed clinically. A sidestream dark field camera (SDF) was used for measurement. Video-quality was evaluated with MIQS (microcirculation image quality score). Both AVA 4.3C- and AVA POEM-software analyzed the videos. Seventeen patients ≥65 years not showing dehydration served as control. RESULTS: Thirteen patients (8 female) were included. The average age was 83±8 years. The mini-mental test was 17±15 points, the Clinical Frailty Scale 4±3, the Barthel-Index 59±39. None of these parameters correlated with MIQS (3.4±4.2 SD (“acceptable”)). Dehydrated patients had a slightly impaired microcirculation, with a significantly lower percentage of perfused small vessels compared to control (83.1±7.7% versus 88.0±6.0%, P < 0.05). After rehydration, there was acute improvement in the microcirculation. CONCLUSIONS: Sublingual microcirculatory SDF-measurement is both, safe and valid for dehydrated old patients - regardless of frailty, age or cognitive performance. Dehydration leads to an impaired microcirculation.