Experimental evidence indicates that tumour metastases can exist for long periods in a dormant state, with cell proliferation balancing cell death. However, this balance can be upset, by removing the primary tumour for instance, which causes the metastasis to grow, or by adminstering a substances inhabiting angiogenesis which causes the metastasis toi regress. A mathematical model is presented for the growth of a tumour metastasis, which by postulating the possibility of a local imbalance between cell proliferation and cell death though apoptosis, is able to explain some of these observations. A prediction of the model is that at any position within the metastasis there will be a radial movement of cells, even in the document state.