Benzoyl peroxide (BP) exerts its therapeutic effect for acne vulgaris through reduction of Propionibacterium acnes. A 1.0 to 2.0 log reduction in P acnes has been demonstrated primarily on the face with use of "leave-on" BP formulations, but also with some BP cleansers. In addition to use for facial acne vulgaris, cleanser formulations of BP are commonly used for truncal acne vulgaris due to ease of use on a large body-surface area and to avoid bleaching of fabric. To date, evaluation of P acnes reduction on the trunk has not been well studied with BP formulations, especially with the use of recognized and standardized methods to accurately determine P acnes colony counts. A previous study demonstrated that a BP 8% cleanser did not reduce counts of P acnes on the back when subjects were instructed to apply the cleanser in the shower, allow it to dry for 20 seconds on the skin, and then rinse off the cleanser. Evaluation of specified time intervals between application on the back and rinsing with BP formulations would help to better define the necessary skin contact time associated with high reductions of P acnes (>90%), recognizing also the potential roles of BP concentration and vehicle. This 2 week study using quantitative bacteriologic cultures evaluates the effectiveness of BP 9.8% emollient foam in reducing P acnes levels on the back with 2 minutes of skin contact time and compares results with a BP 5.3% "leave-on" emollient foam formulation. Short contact therapy utilizing a 2 minute skin contact time with BP 9.8% emollient foam used once daily over a 2 week duration was highly effective in reducing the quantity of P acnes organisms on the back and provided comparable colony count reduction to "leave on" therapy using BP 5.3% emollient foam.