We present a novel microstructure that enables 3 dimensional hydrodynamic focusing of cells in a pressure driven micro cell sorter. This so-called “chimney” structure for single step coaxial sheathing was designed and optimized through simulation, and then successfully micromachined in silicon as part of an integrated micro cell sorter. Coaxial sheathing was clearly demonstrated by velocity distribution experiments and shown to be in accordance with simulation data. The “chimney” structure is easy to machine and can be monolithically integrated with a variety of other microfluidic structures.