Inclusion Criteria:

Patients eligible for enrolment in the study must meet all of the following criteria:

• Males <85 years of age
• No clinically relevant abnormal findings on the screening ECG
• Patients with asymptomatic PSA failure following radical therapy with curative intent for clinically localised prostate cancer. PSA failure is defined as:
• After primary radiotherapy:
• 3 rises in PSA levels from nadir PSA, with each determination at least 4 weeks apart and a final PSA level ≥2 ng/mL above nadir PSA
• Time from radiotherapy should be at least 1 year from termination of radiotherapy treatment
• After radical prostatectomy with or without salvage radiotherapy:
• 3 rises in PSA level from nadir PSA, with each determination at least 4 weeks apart and each PSA level ≥0.2 ng/mL and a final PSA level ≥0.4 ng/mL (nadir PSA is defined as the lowest PSA value achieved after therapy)
• Serum PSA levels:
• ≥2 ng/mL and ≤20ng/mL for primary radiotherapy patients
• ≥0.4 ng/ml and ≤10ng/ml for radical prostatectomy with or without salvage radiotherapy patients
• PSADT >3 months and ≤24 months
• Clinical stage T1-T3a N0 M0
• Non-metastatic prostate cancer, as confirmed on a negative bone scan performed within 6 months prior to randomisation (Visit 2)3.
• No evidence of local recurrence in radical prostatectomy or salvage radiotherapy patients
• Expected survival ≥2 years
• Eastern Cooperative Oncology Group (ECOG) performance status 0, 1 or 2 (see Appendix 1)

Miscellaneous:

• Able to swallow and retain oral medication
• Able and willing to participate in the full 2 years of the study
• Able to read and write (the MAX-PC questionnaire is self-administered), understand instructions related to study procedures and give written informed consent
• In France, a patient will be eligible for inclusion in this study only if either affiliated to or a beneficiary of a social security category.

Exclusion Criteria:

• Any unstable serious co-existing medical condition(s) including but not limited to myocardial infarction, coronary bypass surgery, unstable angina, cardiac arrhythmias, clinically evident congestive heart failure or cerebrovascular accident within 6 months prior to Visit 1, or uncontrolled diabetes or peptic ulcer disease which is uncontrolled by medical management
• Abnormal liver function tests (greater than 2 times the upper limit of normal [ULN] for alanine aminotransferase [ALT], aspartate aminotransferase [AST] or alkaline phosphatase [ALP] or >1.5 x ULN for bilirubin).
• Serum creatinine >1.5 x ULN
• History of another malignancy within 5 years that could affect the diagnosis of prostate cancer
• History or current evidence of drug or alcohol abuse within 12 months prior to Visit 1
• History of any illness (including psychiatric) that, in the opinion of the investigator, might confound the results of the study or pose additional risk to the patient
• Known hypersensitivity to any 5-AR inhibitor or to any drug chemically related to dutasteride

Disease characteristics:

• Serum PSA levels
• >20 ng/mL in primary radiotherapy patients
• >10 ng/mL in radical prostatectomy with or without salvage radiotherapy patients
• PSADT ≤3 months or >24 months
• Biochemical failures in post brachytherapy patients
• Clinical stage N+ or M+
• Patient has previously been treated for prostate cancer with any of the following:
• Chemotherapy
• Oestrogens (e.g. megestrol, medroxyprogesterone, cyproterone, Diethylstilbestrol [DES])
• Drugs with anti-androgenic properties (e.g. spironolactone if >50mg/day, flutamide, bicalutamide, ketoconazole, progestational agents), (except when used for adjuvancy or neoadjuvancy in the context of a primary radical treatment in which case their use should have been for no more than 6 months and should have completed at least 1 year before Visit 1 [Note: the use of topical ketoconazole is permitted prior to and during the study and the use of cimetidine is permitted prior to study entry]
• GnRH analogues (e.g., leuprolide, goserelin) except when used for adjuvancy or neoadjuvancy in the context of a primary radical treatment (in this case use should have been for no more than 6 months and should have finalised at least 1 year before Visit 1)
• Orchiectomy

Concomitant medications:

• Glucocorticoids, except inhaled or topical, are not permitted within 3 months prior to Visit 1 or during the study
• Current and/or previous use of finasteride (Proscar, Propecia) or dutasteride (GI198745, AVODART™) exposure within 6 months prior to Visit 1
• Anabolic steroids within 6 months prior to Visit 1
• Participation in any other investigational or marketed drug trial within the 30 days prior to Visit 1 or any time during the study period