The fully developed laminar flow and heat transfer behavior in periodic serpentine circular-section channels has been studied using computational fluid dynamics (CFD). ne serpentine elements are characterised by their wavelength (2 L), channel diameter (d), and radius of curvature of bends (R-c) and results are reported for (3 < L/d < 12.5, 0.525 < R-c/d < 2.25) and Reynolds numbers up to 200. The flow is characterised by the formation of a single pair of Dean vortices after each bend, these being stronger if the bend is in the same direction as the previous bend. The vortices decay downstream of the bends, but, as the Reynolds number is increased, the flow space is increasingly dominated by these vortices. At higher values of Reynolds number (corresponding to a Dean number of about 150), a second pair of vortices develops. For Re > similar to 200, the flow becomes unsteady. Constant wall heat flux (H2) and constant wall temperature (T) boundary conditions have been examined for a range of fluid Prandtl number (0.7 < Pr < 100). The formation of Dean vortices produces significant heat transfer enhancement relative to flow in a straight pipe, with the effect being greater at higher values of Pr. Pressure drop is also increased but to a lesser extent. The alignment of the flow with the vorticity in these structures allows significant mixing of the fluid without creating large pressure-drop penalties. Dean vortices are also found to inhibit flow separation. These results suggest an effective method for enhancement of heat transfer in deep laminar flows.