Objectives : The purpose of this study is to examine the efficacy and side effects of quetiapine and haloperidol for the treatment of symptoms of delirium. Methods : One hundred and seven patients with delirium were recruited and randomly assigned to receive a flexible-dose regimen of quetiapine or haloperidol over 7days and seventy-seven patients completed the study(quetiapine group N=40, haloperidol group N=37). The severity of delirium was assessed by using Memorial Delirium Assessment Scale(MDAS) scores, the psychiatric and behavioral symptoms were assessed by Neurobehavioral Rating Scale(NRS) scores, and the cognitive status was measured by Mini-mental state examination Korean version(MMSE-K) scores. The side effects were measured by Drug Induced Extrapyramidal Symptoms Scale(DIEPSS) scores. Results : MDAS scores significantly improved in both treatment groups. NRS scores also significantly improved in both treatment group, but the group-by-time effect approached significance, likely caused by the greater decrease in scores of the quetiapine group. MMSE-K scores significantly improved only in the quetiapine group. Side effects associated with treatment were not significant in either treatment groups. Conclusion : This study suggests that quetiapine is as efficacious as haloperidol in the treatment of delirium. In particular, quetiapine seems to improve psychiatric and behavioral problems of delirium and was more effective than haloperidol in cognitive improvement.