Citizens of the European Union cross the borders of the member states every day for different reasons. Some of them often travel to another member state for work, while some of them cross borders and temporarily stay in another member state just for one-day trips or annual vacations of a few days. Regardless of the reason for the trip, during such a temporary stay in another member state, sudden, unplanned health situations may occur that require immediate medical intervention. For example, during a temporary stay in another member state, a person may suddenly fall ill, and as an EU citizen, he has the right to provide necessary health care in the member state of temporary stay, if it cannot be postponed until returning home. This necessary care is provided by the member state of temporary residence according to the national legislation, but at the expense of the competent member state. The injured person has the same rights to treatment as the insured persons of the country in which he is temporarily staying. In such situations, it is important that the injured person show the EHIC. This work covers exactly these cases. It is explained what unplanned treatment is, i. e. necessary health care, and what rights arise from it. It is also described how unplanned treatment is legally regulated, what is i.e. the meaning of Regulation 883/2004 and 987/2009 against Directive 2011/24. At the end of the paper, cross-border health care according to Croatian legislation was also mentioned.