We derive a homogenization expansion approach to obtaining the leaky modes, or scattering resonances, of photonic waveguides whose transverse structures are N-fold rotationally symmetric. Examples include microstructured, or “holey”, optical fibers with air holes arranged in one or more concentric rings. We numerically calculate the leading order correction to the complex effective indexes for the leaky modes of various microstructured photonic waveguides in the scalar approximation. We observe that in many instances this two-term truncation of the homogenization expansion gives very good agreement with full simulations, even for fairly small values of N.