The polymerase chain reaction is now making a major contribution to the diagnosis of infections of the central nervous system. The polymerase chain reaction can increase the sensitivity, specificity and speed of diagnosis of these conditions, and so make a significant impact on their management. In this article we discuss the application of the polymerase chain reaction to the diagnosis of common infections of the central nervous system. Bacterial antigen testing in cerebrospinal fluid is likely to be displaced by polymerase chain reaction.