DISEASE CHARACTERISTICS: Histologically confirmed adenocarcinoma of the prostate or metastatic carcinoma of presumptive prostate origin as manifest by the presence of sclerotic bony metastases and a serum PSA level greater than the upper limit of normal Radiologically proven progressive bone disease (e.g., new bone scan lesions, increased uptake of isotope at previous sites of disease, and/or increasing bone pain) Hormone refractory disease (i.e., disease progression or recurrence despite documented castrate levels of serum testosterone achieved by bilateral orchiectomy or antiandrogen therapy) Bone pain due to metastatic disease Patients must have achieved stable analgesia for at least 7 days No uncontrolled epidural metastases

PATIENT CHARACTERISTICS: Age: Any age Performance status: ECOG 0-3 Life expectancy: At least 3 months Hematopoietic: WBC at least 3,000/mm3 Absolute granulocyte count greater than 1,500/mm3 Platelet count greater than 100,000/mm3 Hepatic: Bilirubin no greater than 3.15 mg/dL Renal: Creatinine less than 2.26 mg/dL Serum calcium no greater than 3.1 mmol/L Cardiovascular: Patients with history of angina pectoris, previous cardiac infarction, hypertension, or valvular or congenital heart disease must have baseline measurement of LVEF exceeding 50% Other: No other malignancy within 5 years except nonmelanomatous skin cancer No active infection or any other contraindication to chemotherapy with mitoxantrone No spinal cord or nerve root compression No unstabilized impending pathological fractures

PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: One previous course of chemotherapy allowed No prior mitoxantrone or other anthracycline Endocrine therapy: See Disease Characteristics At least 4 weeks since prior nonsteroidal antiandrogens Radiotherapy: At least 4 weeks since prior radiotherapy At least 8 weeks since prior strontium-89 or samarium-153 Surgery: Not specified Other: No prior bisphosphonate therapy