Method for determining a parameter which is indicative for whether a patient is delirious or not, or is at risk of becoming delirious or not, wherein the method comprises the steps of: —providing electroencephalography (EEG) data comprising recording signals from at least two electrodes located on different locations on the patient's scalp during a predetermined time period, for instance at least 10 seconds, wherein at least one of the signals is recorded from the frontal half on the scalp; —processing said EEG data for obtaining a deviation signal from the two recording signals from the electrodes; —analyzing said deviation signal in the frequency spectrum for establishing slowing of said deviation signal and defining the parameter as the degree of slowing of said deviation signal which in combination with the locations of the recordings on the patient's scalp is indicative whether said patient is delirious or not, or is at risk of becoming delirious or not.