Until now, the economic implications of nonadherence to drug therapies have been mostly assessed in a cost perspective and understood as the impact of nonadherence behaviours on the cost-effectiveness ratios of particular therapies. In HIV disease, some modelling approaches have suggested that high adherence levels might improve the cost-effectiveness of highly active antiretroviral treatment regimens. However, there is an urgent need for research directly observing and measuring the costs of nonadherence behaviours. In addition to this cost perspective, the authors argue that the economic impact of nonadherence should also be studied in a microeconomic--patient-oriented--perspective. Major implications of this microeconomic perspective are drawn. First, the microeconomic approach highlights that nonadherence may often be a rational choice of the patient, thus suggesting new ways for improving adherence. Second, it implies that the adherence behaviour can be interpreted as an indicator of patients' subjective valuation of highly active antiretroviral treatment therapies; in particular, higher levels of adherence are associated with higher patient well-being. Third, it shows that the adherence behaviour cannot be regarded as an exogenous variable when the impact of adherence on health outcome or on cost-effectiveness ratios is studied, thus requiring the use of specific statistical or econometric methods. [Authors]