The fate of cells that disseminate from a primary breast tumour remains poorly understood. Studies of the kinetics of recurrence in breast cancer patients are raising important questions about the biology of the metastatic process. Where do tumour cells reside once they leave the primary tumour, and what factors influence their dormancy and recurrent growth? Clinical data analyses are leading to hypotheses about the biology of metastasis, dormancy and recurrence. A combined clinical and experimental approach to testing these hypotheses will help to clarify this important issue in breast cancer biology and patient care.