Inclusion Criteria:

• Provision of written informed consent
• A diagnosis of opiate dependence as defined by Diagnostic and Statistical Manual of Mental Disorders- Fourth Edition (DSM-IV) during the past two years.
• A diagnosis of generalized anxiety disorder as defined by Diagnostic and Statistical Manual of Mental Disorders- Fourth Edition (DSM-IV) within the past 6 months.
• Males and females aged 21-55 years
• Female patients of childbearing potential must be using a reliable method of contraception and have a negative urine human chorionic gonadotropin (HCG) test at enrollment
• Able to understand and comply with the requirements of the study
• Subjects must be free of illicit drug use for the past 3 months
• Subjects must have received methadone maintenance therapy for at least 3 months, and have been at least 2 weeks methadone-free
• Good health, as assessed by medical history, physical examination and laboratory tests

Exclusion Criteria:

• Pregnancy or lactation
• Current diagnosis of any Axis I disorder other than GAD, substance dependence in remission, or nicotine dependence
• Patients who, in the opinion of the investigator, pose an imminent risk of suicide or a danger to self or others
• Known intolerance or lack of response to quetiapine fumarate, as judged by the investigator
• Use of any of the following cytochrome P450 3A4 inhibitors in the 14 days preceding enrolment including but not limited to: ketoconazole, itraconazole, fluconazole, erythromycin, clarithromycin, troleandomycin, indinavir, nelfinavir, ritonavir, fluvoxamine and saquinavir
• Use of any of the following cytochrome P450 inducers in the 14 days preceding enrollment including but not limited to: phenytoin, carbamazepine, barbiturates, rifampin, St. John's Wort, and glucocorticoids
• Administration of a depot antipsychotic injection within one dosing interval (for the depot) before randomisation
• Medical conditions that would affect absorption, distribution, metabolism, or excretion of study treatment
• Unstable or inadequately treated medical illness (e.g. diabetes, angina pectoris, hypertension) as judged by the investigator
• Involvement in the planning and conduct of the study
• Previous enrollment or randomisation of treatment in the present study.
• Participation in another drug trial within 4 weeks prior enrollment into this study or longer in accordance with local requirements
• A patient with Diabetes Mellitus (DM) fulfilling one of the following criteria:
• Unstable DM defined as enrollment glycosylated hemoglobin (HbA1c) >8.5%.
• Admitted to hospital for treatment of DM or DM related illness in past 12 weeks.
• Not under physician care for DM
• Physician responsible for patient's DM care has not indicated that patient's DM is controlled.
• Physician responsible for patient's DM care has not approved patient's participation in the study
• Has not been on the same dose of oral hypoglycaemic drug(s) and/or diet for the 4 weeks prior to randomization. For thiazolidinediones (glitazones) this period should not be less than 8 weeks.
• Taking insulin whose daily dose on one occasion in the past 4 weeks has been more than 10% above or below their mean dose in the preceding 4 weeks Note: If a diabetic patient meets one of these criteria, the patient is to be excluded even if the treating physician believes that the patient is stable and can participate in the study.
• An absolute neutrophil count (ANC) of 1.5 x 109 per liter
• Positive urine drug screening test for drugs of abuse