There is reasonable evidence that an objective basis exists for SBS, based on both laboratory and field studies. Intervention strategies for environmental control in the office appear to solve many of the problems, although engineering design strategies sometimes are inadequate. Is there evidence that these data pertain to MCS? A major problem arises from the missing agreed-upon case definitions for the two conditions. They are similar in that both appear to be due to low levels of agents, well below PELs, TLVs, and other levels established by scientific groups. This similarity may be due to one of two separate mechanisms. First, criteria levels do not appear to protect everyone against effects such as irritation, based on empiric field and laboratory data. Second, data do indicate that some individuals respond to concentrations of agents at levels below such "criteria" levels, because of definable and measurable problems such as atopy or more rapid tear film break-up time. Both syndromes appear to affect different organ systems, or at least be associated with symptoms attributed to different organ systems (mucosal irritation, chest symptoms, nausea, headaches). The problem of resolution after leaving work, or the inciting building, is somewhat more difficult, as questionnaire-based responses do not appear to show strong concordance between the two main questionnaires, administered simultaneously, that are used to define SBS symptoms. Psychological aspects clearly influence interpretations of symptoms in SBS. Although this is documented for some proportion of MCS/IEI, it remains controversial. Psychological aspects of discomfort, and stress, are clearly acknowledged to be important in office worker symptoms, at least as pertains to their magnitude. Some years ago, the AMA Council of Scientific Affairs used one publication by this author to distinguish the two conditions. In the absence of more formal study, and better case definitions, the scientific evidence summarized here simply provides evidence supporting a physiologic basis for symptoms at very low levels, the influence of psychological states on symptoms, and the presence of reversible dose-related symptoms at levels below defined criteria levels. Still, this author feels uncomfortable equating the two syndromes, experiences patients who have been labeled MCS/IEI differently, and remains unconvinced that the syndromes are indistinguishable.