Lawn mower (10) height-adjust systems include three embodiments. Two permit the user to adjust the position of the deck (12) relative to each wheel (18) individually. The third permits the user to adjust the positions of all four wheels (18) simultaneously, aided by a counterbalance system. A modification of the third embodiment is available to automatically bypass the height-adjust mechanism in the event an abnormally high load is placed on the deck (12). One of the two individual systems (16) permits an operator to adjust the position of a respective wheel (18) simply by removing a single nut (82) and washer (80) from an axle bolt (78). The second individual height-adjust system permits the operator to adjust the height of an individual wheel (18) by using one hand, and without having to disassemble the mechanism. The system which adjusts all four wheels (18) simultaneously links front and rear axle subassemblies with a connecting rod (306), and indexes all four wheels (18) simultaneously through a release handle (324) which is recessed in a cavity formed in the lawn mower deck (12). When the operator lifts the release handle (324), he or she may simultaneously grasp a portion of the deck (12) with the same hand and easily raise or lower it to a desired position. A single unitary plastic deck (12) is the foundation for each of the three height-adjust systems, thereby significantly reducing the cost otherwise incurred by manufacturing three separate types of lawn mowers.