Recently, we have developed microcomputer-oriented pacemakers with true software flexibility. The first 25 devices implanted (DPG-1 type) exhibited an unexpected phenomenon caused by a microcomputer error. A safety mechanism switched off all special functions and automatically reset all parameters to standard settings. The problem was solved by noninvasive modification of the software. More than 200 units (TX-1 type), which are rate responsive pacemakers based on the QT principal, were implanted up to May 1984. These pacemakers were built using the same microelectronics as used in the DPG 1 pacemaker. Knowledge gained from clinical evaluation was directly applied not only to the patients receiving new implants but also to the preexisting implantees. This was done by software reprogramming in the implanted unit.