Seventy-seven cases of prostate cancer were treated for 5 years at our department and all cases were followed by bone scintigraphy and tumor markers. Of these cases on case of flare response on bone scintigraphy was recognized. A 51-year-old man was hospitalized with chief complaint of lumbago. Serum PAP and gamma-Sm levels were 320 ng/ml and 15 ng/ml, respectively. Prostate biopsy revealed moderately differentiated adenocarcinoma. Bone scintigraphy and CT scan demonstrated multiple bone metastases and lymph nodes involvements. Treatment was started with diethylstilbestrol diphosphate (DES). At one month after the initiation of treatment tumor markers fell down to the normal level and lumbago was diminished, but only serum alkaline phosphatase was elevated and bone scintigraphy showed apparent progression of individual lesions (flare response). The treatment was not altered. At the times after 2, 8, 12 and 36 months successful treatment the bone imaging improved with reduced tracer uptake and no new lesions. The flare response is a healing reaction and is followed apparent improvement. In general, serial bone scintigrams accurately depict the activity of bone metastases in the patients of prostate cancer, but between 1 and 3 months after starting treatment the paradoxical "flare phenomenon" should be taken care.