Spurious hyperphosphatemia was diagnosed in a 6-year-old, neutered female, mixed-breed dog with chronic lymphocytic leukemia associated with an IgM monoclonal gammopathy. The spurious hyperphosphatemia was probably caused by paraprotein precipitation which interfered with the ASTRA 8 automated analyzer measurements. Serial dilutions of the sample did not change the phosphorus value. Another analyzer system in which a protein-free sample was prepared prior to analysis gave a normal serum phosphorus concentration. There was a linear relationship between the amount of paraprotein and the measured total serum inorganic phosphate (r=0.75). A review of 700 chemistry profiles from dogs and cats and a review of 36 cases with polygonal gammopathy and 6 cases with monoclonal gammopathy did not reveal other cases of spurious hyperphosphatemia.