The fiber-coating concentricity of a coated optical fiber is controlled by non-axisymmetrically heating the coating material in the sizing die (40) with respect to the z-axis along which the fiber (47) is drawn. By controlling the temperature profile and thus the viscosity profile of the coating material, the amount of coating material applied to each azimuthal position of the fiber is controlled as it exits the sizing die (40). The apparatus that provides the desired temperature profile can also be employed to control the coating diameter.