The management of health risks in the human environment is usually supported by expert assessment of the risk. We studied two cases of cancer risk in the Netherlands (connected with dioxin and formaldehyde respectively) in which reassessment of the risk was needed shortly after a first assessment. From the change in the assessment in these two cases, we could reach a number of conclusions about the role of expertise in the process of health risk management. First, the experts already include the evaluation of risk in their assessment. Second, they do not seem to be paradigmatically predisposed to adhere to a certain line of argument, but use whatever data are available to enable them to reach conclusions on risk management problems. Experts are seen to act as service engineers, able to use a scientific toolbox to help solve such problems. This `service engineering' model of the role of expertise demonstrates the strong interrelationship and `fine tuning' between experts and policy decisions.