INTRODUCTIONDelirium is characterized by an acute decline in the level of consciousness and cognition with particular impairment in attention. Other associated features include abnormal psychomotor activity, sleep cycle impairment and psychiatric symptoms such as abnormalities of mood, perception and behaviour.1 It develops over a short period of time and fluctuates during the course of the day. It is a common neuro cognitive disorder which heralds an increase in morbidity and mortality.2 It is also known by other terms such as acute confusional state, toxic psychosis and metabolic encephalopathy. 1 Delirium is usually rapid in onset with a diurnally fluctuating course and duration of less than six months. Based on the etiology, DSM 5 classifies delirium as substance intoxication delirium, substance withdrawal delirium, medication-induced delirium, delirium due to another medical condition and delirium due to multiple etiologies.2 It is also further specified as hyperactive delirium, hypoactive delirium and mixed delirium based on the level of psychomotor activity.2,3 The cause of ABSTRACT Background: Delirium is one of the most common causes of referral of medically ill patients for psychiatric management and is associated with significant morbidity and mortality. The aim of the study was to determine the prevalence and etiology of delirium among medically ill patients and to describe its phenomenology. Methods: A cross-sectional descriptive study was conducted in the general medical ward of a tertiary care hospital. Four hundred patients were screened for delirium using the confusion assessment method. The delirium rating scale revised-98 was used to further characterize the severity and phenomenology of delirium. Descriptive statistics such as frequencies, means and standard deviation were used. Chi square test was applied to compare categorical variables and Fisher's exact test was used to test associations. Student t test was used to compare the means of independent groups. Results: The prevalence of delirium was 9.75%. Male gender (69.23%) and age above 60 years (66.67%) were significantly associated with delirium (p values of 0.04 and <0.001 respectively). Mixed delirium was the most common motor subtype (46.15%). The most common cause of delirium was metabolic encephalopathy (43.59%). There was significant association between hyperactive delirium and psychotic symptoms. Delirium went undiagnosed in 61.64% of the cases. Conclusions: Delirium is a common problem among medically ill patients and runs the risk of being undiagnosed. Clinicians should routinely screen patients at high risk for delirium to decrease the morbidity and mortality associated with this dreaded, yet potentially reversible entity.