We measured the transmission and analyzed out-of-plane loss in Fabry–Pérot filters based on photonic crystals etched in a suspended InP membrane. The resonant cavity of the Fabry–Pérot filter is based on a single row line defect introduced in a triangular lattice of air holes. The transmission spectrum of these structures is measured by using the end-fire method. We measured a cavity quality factor of 3200 for a resonance wavelength in the 1.5 μm wavelength range. We identify that radiation through the holes at both extremities of the resonant cavity largely contributes to the loss in the device.