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Objectives: To clarify the effect of intermittent androgen suppression on the time to androgen-independent progression and changes in quality of life (QOL)., Methods: Patients with locally advanced or metastatic prostate cancer were treated with a combination of leuprolide acetate and flutamide for 36 weeks. When the serum prostate-specific antigen (PSA) levels at 24 and 32 weeks were less than 4.0 ng/mL, treatment was withheld until the PSA level reached 15 ng/mL or the pretreatment level. This cycle of on-treatment and off-treatment was repeated until PSA failure (three consecutive increases in PSA level greater than 4.0 ng/mL during the on-treatment period) or symptomatic progression was observed. Changes in QOL were assessed by a self-assessment questionnaire., Results: Forty-nine patients (26 with T3N0M0, 8 with T2-T3N1M0, 2 with T4N0M0, and 13 with T2-T3N0M1) were enrolled. The mean follow-up period was 136.5 weeks. Thirty-one patients finished cycle 1, six finished cycle 2, and three finished cycle 3. The mean off-treatment duration in cycles 1, 2, and 3 was 46.1, 36.9, and 23.3 weeks, respectively. In the off-treatment period, statistically significant improvements in the QOL score were observed in the categories of potency (11.4 versus 2.4) and social/family well-being (20.3 versus 16.1) compared with those in the on-treatment period. PSA failure occurred in 6 patients (3 with T3N0M0 and 3 with T2-T3N1M0), and all patients were alive at last follow-up., Conclusions: Our interim analysis indicated that QOL is remarkably improved during the off-treatment period. Intermittent androgen suppression would be a viable option for treatment of advanced prostate cancer, although a randomized controlled study is required to determine whether intermittent androgen suppression prolongs the time to androgen-independent cancer. We will continue follow-up in this study to a minimum of 3 years.