Helical flows are investigated in structured microchannels with regard to micromixing by means of computational fluid dynamics (CFD). In the case of bas-relief structured channels the numerical results are found to be in good agreement with experimental findings. The magnitude of the transverse flow is computed for various Reynolds numbers and for different geometries including channels with bas-relief structures on two opposite walls. Transverse flows in structured microchannels are compared to secondary flow patterns in curved square channels. The corresponding helical flows are analyzed for Dean numbers ranging from 1 to 900. Special attention is paid to the occurrence of additional vortices close to the center of the outer channel wall. Based on the results a new ope of micromixer is proposed that relies on the transition of the secondary flow pattern from two to four vortices. (C) 2004 American Institute of Chemical Engineers.