The technetium methylene diphosphate bone scan was evaluated in 100 consecutive new patients presenting with carcinoma of the prostate. 48 % of the patients had a positive bone scan at the initial diagnosis. The scan was more helpful than the skeletal X-ray in the diagnosis of bone métastasés: 23% of the X-ray-negative patients were scan-positive. Serial bone scans were more sensitive than either X-rays or serum acid phosphatase in following the progress of the disease. It is concluded that the bone scan in patients with prostatic carcinoma can be used as a reliable tumour marker, especially for monitoring the course of métastasés.