The recent introduction of gray scale ultrasound, which uses a higher power output, has made it more difficult ot assess the cystic or solid nature of sonolucent mass lesions using the cirteria developed for bistable imaging, i.e. the amount of through transmission and the strength of the posterior wall echo. However, a cyst may be reliably diagnosed if a totally echofree area is seen adjacent to an area with low-level echoes derived from tissue parenchyma. We have termed this the "light bulb sign." Visualization of the sign facilitates the decision of the optimal time for abscess drainage.