The polymerase chain reaction (PCR) is an extraordinary tool for studying cancer. It is both more sensitive and more rapid than any other detection method, and can be performed without special expertise. It is ideal for the detection of minimal residual disease, particularly in lymphoid malignancies, but also for any cancer with a genetic abnormality that is defined at the molecular level. Other uses, such as diagnosis of cancer based on detection of translocations or point mutations, have an intriguing potential, but remain investigational. At this point, few clinicians would be willing to base a diagnosis, let alone a treatment plan, on a diagnosis based on PCR findings without confirming histology.