A keyless electronic combination lock is adaptable for placement on a door, and operates independently of any other locks and without a central control system. The lock has a keypad for entering combinations, a random access memory (RAM) into which the entered data is loaded, a microprocessor, and an electrically erasable programmable read only memory (EEPROM). One or more entry codes for opening the lock are stored in the EEPROM and loaded into the RAM when the lock is powered-up. To open the lock, one of these codes or a master entry code must be entered at the keypad. Each entry code can be individually prevented from opening the lock by entering a corresponding individual lock-out code. All of the entry codes can be locked-out by entering a master lock-out code, by entering all of the individual lock-out codes or by throwing a dead bolt. The master entry code works regardless of the lock-out status and regardless of the dead bolt. To change lock-out status, the lock-out code is simply reentered. All of the codes can be changed as desired by entering a programming code, followed by an identifying code to tell the microprocessor which code to change. The programming code can be installed in the lock at the time of manufacturing the lock, and it can be changed during use in the field.