The evolution of a shear flow with an imbedded streamwise vortex is considered. An idealized model for the vortical structure is used; the vortex is assumed to be of infinite extent in the stream direction, and to be a potential vortex (vortex filament) turned on at time zero, and subsequently allowed to diffuse under the action of viscosity. The ambient flow is taken to be, initially, a linear shear profile; the flow then evolves under the joint action of viscosity and convection induced by the vortex. Boundaries are assumed to be infinitely removed from the vortex core. A similarity variable is found which reduces the equation for the induced streamwise velocity perturbation to an ordinary differential equation, which is easily solved numerically. The vortex Reynolds number, circulation/viscosity, is found to be of prime importance. Calculated velocity profiles are presented.