Bone metastases frequently occur in prostate carcinoma. Total body radionuclide scan with diphosphonate methylene labelled with 99Tc is commonly used to diagnose such metastases. However this technique is aspecific and frequently unreliable. In recent years several biological markers dealing with bone metabolism were studied. Serum determination of skeletal alkaline phosphatase (ALP) and moreover of its bone isoenzyme (BAP) could be considered a reliable index of osteoblastic activity. In this preliminary report we analyzed a group of 43 patients affected by prostate carcinoma with or without bone metastases. The American Urological Association (AUA) staging system was adopted. Sixteen patients were D2, bone metastases had been suspected by means of radionuclide bone scan and confirmed by Computerized Tomography and/or aimed X-rays. Tandem R-Ostase by Hybritech was used to measure BAP, normal value is set to 20 micrograms/L. All D2 tumours had pathological BAP values (mean value 87.50 micrograms/l); 1/3 stage A, 5/13 stage B, 5/9 stage C and 0/2 stage D1 patients had pathological findings. One of this patients, stage C, revealed a bone metastase at a later bone scan.