An improved electronic lock system is provided for use with real estate lock boxes. Each user has an identification card with a non-volatile secure memory for exchanging data with the lock box, and for exchanging data with a central computer. The user first inserts the card into a connector at the lock box, or at the central computer. The lock box or central computer must first enable (or unlock) the data in the card memory, and then can read the data stored in that card's memory and record this information in lock box memory. The card must then identify itself, and the user must identify himself/herself to the lock box or central computer. After the identification information is authenticated, the user can enter commands to the lock box; e.g., an access code is manually keyed into the lock box keypad by the user to obtain access to a secure compartment.