OBJECTIVE: Acute hospitalization of older patients may be associated with loss of muscle strength and functional performance. The aim of this study was to investigate the effect of acute hospitalization as a result of medical disease on muscle strength and functional performance in older medical patients. DESIGN: Isometric knee-extension strength; handgrip strength; and functional performance, that is, the Timed Up and Go test, were assessed at admission, at discharge, and 30 days after discharge. Twenty-four-hour mobility was measured during hospitalization. RESULTS: The mean (SD) age was 82.7 (8.2) years, and the median length of stay was 7.5 days (interquartile range, 4.25-11). Knee-extension strength did not change over time (1.0 [N·m]/kg, 1.1 [N·m]/kg, and 1.1 [N·m]/kg, P = 0.138), as did handgrip strength (24.2 kg, 23.3 kg, and 23.5 kg, P = 0.265). The Timed Up and Go test improved during hospitalization, from 17.3 secs at admission to 13.3 secs at discharge (P = 0.003), but with no improvement at the 30-day follow-up (12.4 secs, P = 0.064). The median times spent in lying, sitting, and standing/walking were 17.4 hrs per day, 4.8 hrs per day, and 0.8 hrs per day, respectively. CONCLUSIONS: Muscle strength did not change during hospitalization and 30 days after discharge in the acutely admitted older medical patients. Despite a low level of mobility during hospitalization, functional performance improved significantly during hospitalization, without further improvement.