We report an experimental study of broadband sum-frequency generation of a nonselective Q -switched CO laser (pulse duration, ~0.3 μs; repetition rate, ~90 Hz) in ZnGeP 2 crystals with and without an antireflective interference coating. The uncoated crystal surface is found to be optically damaged at a laser radiation intensity of 0.033 GW/cm 2 . Under the same conditions, no damage to the antireflection-coated surface of the crystal is observed. The maximum efficiency of broadband sum-frequency generation of the CO laser in the antireflection-coated sample is 4.8% and turns out to be two times higher than in the uncoated sample. The spectral characteristics of the radiation at sum frequencies do not change in using antireflection-coated and uncoated samples.