ANNOTATION: This paper examines the efficiency of forest biomass used for energy. The study used net energy ratios to compare how much energy is consumed by mechanized harvesting systems of small diameter trees that are transported and used for forest biomass energy to the amount of energy (in BTUs) the forest biomass produces. The overall conclusion showed that forest biomass energy has an encouraging net energy ratio compared to other biomass sources, however increases in miles traveled and diesel fuel costs can inhibit the use of forest biomass for energy. ABSTRACT: What amount of extra energy can be generated after subtracting the total energy consumed to produce the biomass energy? Knowing the ratio between energy output and input is a valid question when highly mechanized systems that consume fossil fuels are used to harvest and transport forest biomass for energy. We estimated the net energy generated from mechanical fuel reduction thinning treatments on pure ponderosa pine stands in Arizona. The mechanized system (felling, skidding, loading, grinding, and hauling) was monitored for energy consumption. Potential energy output from harvested forest biomass was calculated based on hog fuel moisture content and heating value. A 9-day study showed positive net energy output of 3,471,376,292 BTUs. The net energy ratio between energy output and input was 10.41, and the energy cost from stump to energy plant was $4.65/million BTUs. Energy used for hauling hog fuel represented the largest part (36.27%) of the total energy input. The net energy ratio decreased 0.11 with each additional transportation mile. Energy cost increased by $0.04/million BTUs, $0.13/million BTUs, and $1.48/million BTUs for each additional mile of highway, unpaved road, and spur road, respectively. A one dollar per gallon diesel price increase added $0.34/million BTUs to the energy cost. Forest biomass energy has an encouraging net energy ratio compared with other biomass sources, but the high production cost and energy cost can impede the use of forest biomass for energy.