Invasiveness of temperature monitoring devices is presently one of the most serious limitations to the application of oncological hyperthermia (HT). A promising approach aims at detecting temperature variations by monitoring the mean grey level (MGL) of the ultrasonographic image of the tissue. Gaseous ultrasound contrast agents (UCA), enhancing Ultrasonic (US) imaging, are expected to be sensitive to temperature, and are therefore a good candidate as temperature monitoring medium. The present study evaluates the 'in vitro' temporal and thermal stability and the correlation between temperature and MGL using a gaseous UCA (SonoVue) as phantom. No statistical differences were detected between the MGL value of the phantom kept at 43.5 degrees C before (215.2+/-3.5) and after 1 h (214.8+/-2.5), showing good stability at HT temperatures. Data of MGL image vs. temperature were obtained during both heating and cooling experiments in the HT range (30-43 degrees C). A good linearity of MGL vs. temperature (R2=0.976) was found with a good accuracy (2.5%) and a sensitivity of about 6.6 MGL/degrees C.