We present the spectral properties of multiline guided-mode resonance filters designed with extremely thick dielectric films. We treat a dielectric membrane in air with a subwavelength grating inscribed into one surface. As the film is very thick on the scale of the wavelength, it supports a large number of resonant modes. In general, the resonant modes yield a dense reflectance spectrum with irregular appearance. We show that by placing an antireflection layer on the backside of the slab, the interference between the directly transmitted zero order and the diffracted order generating the waveguide modes is eliminated. Thus, a well-shaped, unperturbed comb-like spectrum is realized. A titanium dioxide membrane that is 500 μm thick generates a spectrum with more than 1000 channels separated by ~0.8 nm near the 1.55 μm wavelength.