A 63-year old man was seen in the outpatient clinic because of renal cancer and elevation in bone alkaline phosphatase measured by monoclonalantibodies assay. Known causes of bone isoenzyme augmentation, including bone metastases, were ruled out. The tumoral origin of the isoenzymewas diagnosed because after removal of the tumor the enzymatic levels fell sharply. Several alkaline phosphatase isoenzymes can be producedand secreted by tumors as a paraneoplasic manifestation and their elevation could be a manifestation of an occult neoplasia. Furthermore themonitoring of their blood levels can be useful means of treatment response and a tool to monitoring recurrence if a sharp decrease after removalof the tumor is observed