Fully developed laminar flow and heat transfer in three-dimensional, streamwise-periodic sinusoidal channels with circular and semi-circular cross-sections are considered. Computational fluid dynamics (CFD) is used to investigate the effect of Reynolds number (5 <= Re <= 200) and amplitude to half wavelength ratio (0.222 <= AIL <= 0.667) on heat transfer enhancement and pressure drop for steady, incompressible, constant property, water (Pr =6.13) flows in geometries with L/d =4.5 for the constant wall heat flux (H2) and constant wall temperature (T) boundary conditions. The flow field in the sinusoidal geometries is increasingly dominated by secondary flow structures (Dean vortices) with increasing Reynolds number and AIL. These vortices act to promote convective heat transfer enhancement, resulting in high rates of heat transfer and low pressure loss relative to fully developed flow in a straight pipe. Heat transfer enhancement exceeds the relative pressure-drop penalty by factors as large as 1.5 and 1.8 for the circular and semi-circular cross-sections, respectively. (c) 2006 Elsevier Ltd. All rights reserved.