Reactive oxygen species produced at toxic levels are damaging species. When produced at sub-toxic levels, however, they are involved as second messengers in numerous signal transduction pathways. In addition to these findings, we can add the concept that iron (often viewed as the "villain" in free radical biology) can also be considered as a signalling species. Iron is intimately involved in the regulation of its own storage, compartmentalization and turnover. During adult respiratory distress syndrome (ARDS) and sepsis, such regulation may be aberrant or altered in some predisposed way. Such changes may have profound implications for tissue damage, and for the modulation of the inflammatory response in these patients. The search for a genetic predisposition in patients that leads to the development of ARDS associated with abnormalities in iron turnover and signalling would seem to be an important and logical progression for studies into the disease. These may lead eventually to the design of effective treatment regimens that involve the control of iron.