The investigators designed a randomised multicenter clinical trial for patients with advanced cancer who are admitted to the medical oncology ward or high-care hospice. On admission all patients with advanced cancer will be asked to participate in this study. Consenting patients will be submitted to delirium observation screening according to the DOS. Subsequently DOS screening will be performed twice weekly until discharge. Each patient who's score is > 3 (DOS positive) is showing significant symptoms of delirium and will be submitted to the revised Delirium Rating Scale (DRS-R-98) to confirm diagnosis. To test validity of the DOS scale for this particular population, each DOS positive score will be randomly matched with a patient with a DOS score < 3 (DOSnegative) and this patient will also be submitted to DRS-R-98. When diagnosis of delirium is confirmed by DRS-98, patients will be randomised between treatment of delirium with olanzapine or haloperidol (usual care). Treatment in both groups will consist of identification and management of underlying aetiologies of delirium if possible and adding neuroleptic medication for symptom control. Patients who recover from their delirium episode as well as their caregivers will be asked to complete the Delirium Experience Questionnaire (DEQ) to assess recall of the delirium experience and the degree of distress related to the delirium episode.

Inclusion Criteria:

• Patient has been diagnosed with advanced cancer
• Age ≥ 18
• Patient or his / her significant other speaks Dutch fluently

Exclusion Criteria:

• Delirium is due to alcohol withdrawal
• Patient has been diagnosed with glaucoma, Parkinson's disease or dementia
• Patient is being treated with other neuroleptic medication or lithium
• Patient has another psychiatric disorder that is considered (by investigator) to interfere with assessment of delirium
• Patient had a QTc-interval of > 480 msec on ECG made on admission to the medical oncology ward (ECG is not required if patient is admitted to a high-care hospice)
• Patient has a history of neuroleptic malignant syndrome
• Patient has a history of convulsions.