Quantitative bone scintigraphy was performed in 24 patients with prostatic carcinoma before orchiectomy and up to one to four years after operation. The gamma camera count rate was recorded over the lower thoracic and all lumbar vertebrae 4 h after injection of 99mTc-MDP. Twelve patients had normal bone scintigrams throughout the study. They showed from two years after operation a slight increase in count rate values compared with the preoperative values, probably due to hormonal changes after orchiectomy and to age-related alterations in skeletal metabolism. Twelve patients had abnormal bone scintigrams. They showed as a response to treatment the flare phenomenon with an increase in count rate over the abnormal vertebrae when measured two weeks after operation followed by a decrease after two months. The lowest count rate values were obtained between six months and one year after operation. Thereafter the count rate seemed to remain on the same level. An increase in count rate was connected to progression of disease and the patients died of prostatic carcinoma within one year thereafter.