Lateral scintillation scanning of the knees after intravenous administration of technetium Tc 99m was performed in two patients in whom the differential diagnosis between Baker's cyst and sural thrombophlebitis was considered. One of them turned out to have a Baker's cyst, while the other had a large popliteal fat pad and thrombophlebitis of small veins overlying the sural nerve. These findings correlated well with the findings made on the respective scintillation scans.