For psychologists it is sometimes necessary to measure people's reaction times to the nearest millisecond. This article describes how to use the PC parallel port to receive signals from a button box to achieve millisecond response time accuracy. The workings of the parallel port, the corresponding port addresses, and a simple Linux program to control the port are described. A test of the speed and reliability of button box signal detection is reported. If the reader is moderately familiar with Linux, then this article should provide sufficient instruction for them to build and test their own parallel port button box.