Delirium is a common cause of acute brain dysfunction in patients treated in the Intensive Care Unit (ICU). Aim: The aim of the present study was to investigate the incidence of delirium in the ICU, to establish risk factors for its development and to determine the effect of delirium on patient's length of the stay and mortality in the ICU. Material and Methods: The sample studied consisted of 122 patients hospitalized in the ICU of a General Hospital in Attica, having completed 48 hours of stay. In order to diagnose delirium the CAM-ICU delirium scale was used. There were recorded the demographic characteristics of the sample studied, the medical history, the type of sedation, the severity of their illness during admission, the complications, the environment and psychological factors. Moreover, the length of stay, morbidity and mortality of patients were recorded. Data analysis was performed with the statistical package SPSS-ver.17. Results: 62% (n=76) of the sample studied were male. The mean age of the sample was 57±18 years. Intubation and mechanical ventilation was applied in 90% (n=110) of the studied population and seductive drugs in 90% (n=110) of the sample. Delirium frequency was 43%. Risk factors, according to the results, seems to be arterial hypertension (p= 0.009), smoking history (p=0.023), alcohol abuse (p=0.005), severity of illness in admission Apache II (p=0.033). The age and length of stay in ICU doesn't seem to affect delirium development in ICU. Finally, mortality was clearly increased (p=0.001). Conclusions: The increased frequency of delirium in ICU patients requires measures to prevent it. Factors that seem to be related to delirium development are: arterial hypertension, alcohol abuse, smoking, hyperpyrexia and the usage of sedative drugs., Delirium, also, increases mortality in ICU patients.