We show that a thin disk illuminated by a central source will produce single-peaked broad emission lines if there is a wind emerging from the disk. Line-driven winds in active galactic nuclei and in high-luminosity cataclysmic variables afford two examples of such a situation. The radial velocity of the emitting gas is much smaller than the rotational velocity, but the radial velocity gradients are similar. The large gradient in the radial velocity allows photons to escape more easily along lines of sight where the projected velocity is small. As a result, the lines are single peaked, in spite of the fact that the emission comes from gas on essentially circular orbits. We compare the predicted profiles with the C IV and C III] lines seen in quasars. We suggest a solution to the puzzle of single-peaked Balmer lines in eclipsing nova-like cataclysmic variables.