Microfluidic flow cytometers currently analyze far fewer parameters than conventional flow cytometry or fluorescence activated cell sorting (FACS) in order to minimize cost and complexity. There is a need for microfluidic devices that analyze more and or new cell parameters with compact and minimal means. Here we show a new and explicitly microfluidic parameter, “hydrodynamic” cell size, and compare it to forward scatter in conventional flow cytometry. The hydrodynamic size of cells is determined by the degree of lateral displacement experienced while traveling through a 1.2-mm-wide non-clogging array of micro-fabricated obstacles. We show comparable size resolution between the microfluidic device and forward scatter in conventional flow cytometry and without the need to lyse red blood cells. We use the device to differentiate healthy lymphocytes from malignant lymphocytes by size alone and we use the device to detect increased numbers of activated lymphocytes in blood as a result of exposure to staphylococcal enterotoxin B (SEB), a potential bioterror agent. Together the results demonstrate a microfluidic device that performs some of the measurement and separation tasks of a flow cytometer but at a potentially lower cost and complexity.