Background: Short-stay units are hospital units that provide short-term care for selected patients. Studies have indicated that short-stay units might reduce admission rates, time of hospital stays, hospital readmissions and expenditure without compromising the quality of care. Short-stay units are often defined by a target patient category, a target function, and a target time frame. Hypothetically, short-stay units could be established as part of any department, but this review focuses on short-stay units that provide care for participants with internal medicine diseases and conditions. Objectives: To assess beneficial and harmful effects of short-stay unit hospitalisation compared with usual care in people with internal medicine diseases and conditions. Search methods: We searched CENTRAL, MEDLINE, Embase, three other databases and two trials registers up to 13 December 2017 together with reference checking, citation searching and contact with study authors to identify additional studies. We also searched several grey literature sources and performed a forward citation search for included studies. Selection criteria: We included randomised trials and cluster-randomised trials, comparing hospitalisation in a short-stay unit with usual care (hospitalisation in a traditional hospital ward or other services). We defined a short-stay unit to be a hospital ward where the targeted length of stay in hospital for patients was five days or less. We included both multipurpose and specialised short-stay units. Participants were adults admitted to hospital with an internal medicine disease or condition. We excluded surgical, obstetric, psychiatric, gynaecological, and ambulatory participants. Trials were included irrespective of publication status, date, and language. Data collection and analysis: We used standard methodological procedures expected by Cochrane. Two review authors independently extracted data and assessed the risk of bias of each included trial. We measured intervention effect sizes by meta-analyses for two primary outcomes, mortality and serious adverse events, and one secondary outcome, hospital readmission. We narratively reported the following important outcomes: quality of life, activities of daily living, non-serious adverse events, and costs. We used risk ratio differences of 15% for mortality and of 20% for serious adverse events for minimal relevant clinical consideration. We rated the certainty of the evidence and the strength of recommendations of the outcomes using the GRADE approach. Main results: We included 19 records reporting on 14 randomised trials with a total of 2872 participants. One trial was ongoing. Thirteen trials evaluated short-stay unit hospitalisation for six specific conditions (acute decompensated heart failure (one trial), asthma (one trial), atrial fibrillation (one trial), chest pain (seven trials), syncope (two trials), and transient ischaemic attack (one trial)) and one trial investigated participants presenting with miscellaneous internal medicine disease and conditions. The components of the intervention differed among the trials as dictated by the trial participants' condition. All included trials were at high risk of bias. The certainty of the evidence for all outcomes was very low. Consequently, it is uncertain whether hospitalisation in short-stay units compared with usual care reduces mortality (risk ratio (RR) 0.73, 95% confidence interval (CI) 0.47 to 1.15) 5 trials (seven additional trials reporting on 1299 participants reported no deaths in either group)); serious adverse events (RR 0.95, 95% CI 0.59 to 1.54; 7 trials (one additional trial with 108 participants reported no serious adverse events in either group)), and hospital readmission (RR 0.80, 95% CI 0.54 to 1.19, 8 trials (one additional trial with 424 participants did not report results for participants)). There was not enough information to confirm or refute that short-stay unit hospitalisation had relevant effects on quality of life, activities of daily living, non-serious adverse events, and costs. Authors' conclusions: Overall, the quantity and the certainty of the evidence was very low. Consequently, it is uncertain whether there are any beneficial or harmful effects of short-stay unit hospitalisation for adults with internal medicine diseases and conditions - more trials comparing the effects of short-stay units with usual care are needed. Such trials ought to be conducted with low risk of bias and low risks of random errors to improve the overall confidence in the evidence.