An improved electronic lock system is provided for use with real estate lock boxes where there is the need for many people to access the secured compartment of the lock box in a controlled manner. Each user has an identification card with a non-volatile secure memory (known as a "smart card"), for exchanging data with the lock box, and with a portable computer capable of reading data from the smart card; or a cell phone can be used to gain access information from a central computer. The user first inserts the smart card into the connector attached to the lock box; the lock box reads the user's ID stored in the smart card memory and records this information in lock box memory. The lock box then transfers its access code information and other data to the smart card for further processing. The user then inserts the smart card in a portable card reader to learn the access code, or calls a central computer via a mobile phone system and interacts with the computer to elicit the necessary access code information. The access code is then manually keyed into the lock box keypad by the user to obtain access to the secure compartment. Other modes of operation include an "access token mode" and a "card only mode."