The role of macrophages in tumour growth and development is complex and multifaceted. Whilst there is limited evidence that tumour-associated macrophages (TAMs) can be directly tumouricidal and stimulate the anti-tumour activity of T cells, there is now contrasting evidence that tumour cells are able to block or evade the activity of TAMs at the tumour site. In some cases, tumour-derived molecules even redirect TAM activities to promote tumour survival and growth. Indeed, evidence has emerged for a symbiotic relationship between tumour cells and TAMs, in which tumour cells attract TAMs and sustain their survival, with TAMs then responding to micro-environmental factors in tumours such as hypoxia (low oxygen tension) by producing important mitogens as well as various growth factors and enzymes that stimulate tumour angiogenesis. This review presents evidence for the number and/or distribution of TAMs being linked to prognosis in different types of human malignancy. It also outlines the range of pro- and anti-tumour functions performed by TAMs, and the novel therapies recently devised using TAMs to stimulate host immune responses or deliver therapeutic gene constructs to solid tumours. Copyright © 2002 John Wiley & Sons, Ltd.