DISEASE CHARACTERISTICS: Histologically or cytologically confirmed adenocarcinoma of the prostate Stage IV Evidence of progression despite standard hormonal management including antiandrogen withdrawal, defined as 1 of the following: Development of new metastatic lesions Increase in cancer-related pain A 50% rise in prostate-specific antigen (PSA) levels confirmed by 2 measurements at least 2 weeks apart PSA at least 5 ng/mL Testosterone less than 50 ng/mL

PATIENT CHARACTERISTICS: Age: 18 and over Performance status: ECOG 0-1 Life expectancy: At least 3 months Hematopoietic: WBC at least 3,000/mm3 Neutrophil count at least 1,500/mm3 Platelet count at least 100,000/mm3 Hepatic: Bilirubin no greater than 2.0 mg/dL Renal: Creatinine no greater than 1.3 mg/dL Calcium no greater than 10.5 mg/dL Phosphorus no greater than 4.2 mg/dL No kidney stones within the past 5 years No history of cancer-related hypercalcemia Cardiovascular: No uncontrolled heart failure Other: No other malignancy within the past 5 years except nonmelanoma skin cancer No significant active medical illness that would preclude study Fertile patients must use effective contraception

PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: No more than 1 type of prior chemotherapy for prostate cancer No prior carboplatin or cisplatin for prostate cancer Endocrine therapy: See Disease Characteristics At least 4 weeks since prior flutamide or nilutamide (6 weeks for bicalutamide) Concurrent primary hormonal therapy allowed (e.g., gonadotropin-releasing hormone agonist or antagonist) Radiotherapy: At least 1 month since prior radiotherapy At least 2 months since prior strontium chloride Sr 89 or samarium Sm 153 lexidronam pentasodium Surgery: Concurrent orchiectomy allowed Other: At least 7 days since prior thiazide diuretic At least 30 days since prior investigational therapy No prior calcitriol for prostate cancer No concurrent magnesium-containing antacids, bile-resin binders, or calcium supplements