Generating hydrodynamic/acoustic cavitation (Gogate et al.) for a useful purpose encompasses the following energy transformation steps: (1) Burning fuel in boiler. (2) Expanding the steam in a turbine generator. (3) Using electrical output from the generator to: (a) Pump water through a mechanical constriction (hydrodynamic cavitation), or (b) Operate ultrasonic transducer (acoustic cavitation). The combined loss, which is the mathematical product of each of the above step efficiencies, lowers the overall efficiency. An energy efficient method for generating useful cavitation is presented here. It suggests direct injection of steam (I) into sub-cooled water to produce cavitation, thereby eliminating the inefficiencies of the remaining steps (II and III). Cavitation produced by this technique was experimentally and numerically shown to produce collapse conditions similar to hydrodynamic/acoustic cavitation. Direct steam injection cavitation coupled with acoustic cavitation, exhibited 4-16 times greater energy efficiency as compared to acoustic cavitation alone. Similar effects have been numerically speculated for hydrodynamic cavitation. (C) 2008 American Institute of Chemical Engineers.