We reviewed laboratory and clinical studies bearing on the non-cancer health effects of diesel exhaust (DE) published since the 2002 release of the US EPA Health Assessment Document for Diesel Engine Exhaust. We critically evaluated over 100 published articles on experimental research, focusing on their value for predicting the risk of non-cancer health effects in humans exposed to DE. Human controlled-exposure studies provide new evidence of lung inflammatory effects and thrombogenic and ischemic effects of inhaled DE, albeit for older-model diesel engines and concentrations that are much higher (approximately 300 microg/m(3)) than typical ambient or even occupational levels. Recent animal studies provide insight into the potential mechanisms underlying observed respiratory and cardiovascular health responses; however, because of unrealistically high DE concentrations, the mechanisms elucidated in these studies may not be relevant at lower DE exposure levels. Although larger in number, and suggestive of possible mechanisms for non-cancer health effects at elevated DE levels, interpretation of this recent group of clinical-study findings and laboratory-animal results remains hindered by inconsistencies and variability in outcomes, potentially irrelevant DE-exposure compositions, limitations in exposure protocols and pathways, and uncertainties in extrapolation and generalization. A mechanism of action that allows reliable prediction of adverse health effects at DE-exposure levels typical of the present-day ambient and occupational environment has not emerged. Because of changing diesel-engine technology, inhalation studies using realistic environmental and occupational exposures of new-technology diesel exhaust are of critical importance.