Recent epidemiological evidence points to a link between non-insulin dependent diabetes mellitus type II (NIDDM) and cancer of the colon, liver, pancreas, breast and endometrium. This appears to be due to the long period of hyperinsulinaemia which precedes the clinical phase of NIDDM, insulin promoting colon tumour development as well as acting as a hepatocarcinogen. Indeed, the hormone could play a central role in neoplasia, and its influence could explain the observed enhancing effects of obesity and a high fat diet, as well as the inhibition associated with physical exercise, dehydroepiandrosterone administration and high soluble fibre intake. Measures to decrease insulin levels, including lifestyle improvement and supplementation with agents known to decrease insulin resistance may therefore offer a general approach to prevention of cancer in a wide variety of organ sites of major clinical importance.