Medical short stay units help to increase patient fl ow and decrease length of stay, but selecting appropriate patients for admission to such units is diffi cult. The selection tool used in our unit was effective but cumbersome to apply. We collected prospective data on 297 unselected emergency medical admissions and developed a new scoring system based on four key variables using regression analysis. The model predicted a length of stay of <72 h with an area under the receiver operating characteristic curve of 0.68. The model was then used to select patients for admission to the short stay unit in our trust. Length of stay on the short stay unit had decreased by an average of 2.73 days with our original selection tool, but remained unchanged at an average of 3.02 days using the new simpler tool (p>0.05). This model could now be adopted by other units.