The waveguiding effect on modal gain in narrowly confined structures in optical waveguide devices, such as semiconductor laser diodes, is discussed. It is shown that a difference exists between the rigorous modal gain derived from the wave equation and the approximate modal gain used in the literature. This difference means that there is an enhancement of the rigorous modal gain, which depends on active region sizes. The extent of the enhancement is numerically estimated and found to be nonnegligible in standard optical waveguide devices. >