This is a randomised, double blind, double dummy, multicentre, parallel-group study to compare the efficacy and safety of quetiapine and lithium used as monotherapy in the treatment of mania in patients hospitalised for an acute manic episode. After given of informed consent and undergoing screening procedures, the patients will be randomised into quetiapine or lithium group on Day 1. The efficacy of study treatment on symptoms of mania will be assessed at Day 28. Patients will not permitted to use any psychoactive or antipsychotic medications throughout the study period other than those expressly permitted by the protocol. The patients are required to be hospitalised for the treatment and assessment defined in the protocol. He/She could be discharged from the hospital after Week 2 (i.e. On Day 15) if the investigator believes that it will be clinical appropriate to discharge the patient, that the patient is not suicidal or homicidal, and that the patient could reasonably be expected to continue in the study on an outpatient basis. The patients discharged after Day 15 will be given sufficient study medication for the period from discharge to the next visit. At each centre, the same individual will administer a specific psychiatric assessment for a patient at all study visits in order to reduce variability in rating scale scoring. Before the initiation of the study, a consistency assessment will be done among the investigators who conduct the scale assessment in each centre.