Two patients originally diagnosed as having cellulitis involving the dorsum of the foot actually had bacterial arthritis of an underlying joint. In both patients, even after pyarthrosis was suspected, the wrong joint was aspirated. The arthropathies were located by subsequent 99mTc-phosphate bone imaging at a time when roentgenograms were normal. Early diagnosis and aggressive therapy of septic arthritis are essential to prevent joint destruction and osteomyelitis. Radionuclide bone imaging can identify inflammatory joint disease but it cannot specify etiology. In our patients, however, the differential diagnosis was between skin and joint infection. Radionuclide imaging was of great help in making this distinction.