Successful cancer therapy is intended to eliminate all traces of cancer from the patient. If cancer cells survive the therapy, they may eventually grow back, resulting in relapse. Numerous studies, especially in the setting of hematological malignancies, have shown that the presence of even small numbers of residual cancer cells can increase the risk of relapse. Thus, the development of sensitive methods to detect minimal residual disease may help predict which patients are at risk of relapse, and the hope is that therapies will be developed to eliminate residual cancer cells. Dr. Daniel Sabath, associate professor in the University of Washington department of laboratory medicine, explores the concept of minimal residual disease in cancer, reviews the methods used to detect small numbers of cancer cells, and discusses new technology for detecting minimal residual disease in breast cancer. (Series: Laboratory Medicine Grand Rounds)