A combustion-powered fastener-driving tool includes a combustion-powered power source having a combustion chamber, a reciprocating piston and driver blade, and a valve sleeve reciprocable relative to the power source between a rest position and a firing position. The valve sleeve partially defines the combustion chamber. A lockout device is in fluid communication with the combustion power source and includes a reciprocating gas piston connected to a latch in operational proximity to the valve sleeve. The lockout device is configured such that upon combustion in the combustion chamber, gas from the combustion engages the gas piston and moves the latch to an engaged position in which the valve sleeve is prevented from moving to the rest position.