The effect of underwing frost on a transport aircraft airfoil in a takeoff configuration was studied. Underwing frost can occur when the lower surface of the wing is cooled by fuel cold-soaked in the wing tanks during cruise. Frost may accrete on the wing lower surface while the aircraft is awaiting takeoff. A two-dimensional test was performed in the NASA Langley Low-Turbulence Pressure Tunnel on a representative high-lift airfoil with a leading-edge slat and trailing-edge flap. Frost was simulated on the lower surface using distributed roughness particles. The test was conducted at M = 0.2 and Re = 5 x 10⁶ to 1.6 x 10⁷. The effects of the frost on performance were generally small, with the largest effects occurring for the open-slat case with the frost starting at 12% chord. In this situation, it was found that the frost contaminated the upper surface boundary layer at high angles of attack, increasing drag and reducing maximum lift.