There have been a number of claims that the MOS SF-36 mean scores can be used to discriminate between healthy and non-health persons and that it can also discriminate various levels of health. OBJECTIVES: The purpose of this study was to evaluate the ability of the SF-36 to predict whether or not respondents reported physician determined illness and whether or not respondents perceived themselves to have health problems. METHODS: We used Structural Equation Modeling (SEM) techniques to evaluate the SF-36 and its ability to discriminate between those who reported health problems or reported physician determined illness and those who did not in a sample taken from the 1990 National Survey of Functional Health Status (NHS). RESULTS: The correlation between physician determined illness and Physical Health was - .404, resulting in 16.32 % shared variance. The correlation between reported health problems and Physical Health was - .360, resulting in 12.96 % shared variance. These correlations are markedly lower than those to the 8 first-order scales or between Physical and Mental Health (r = .889). Mental Health could not predict physician determined illness or reported health problems independent of Physical Health. CONCLUSIONS: The SF-36 is relatively poor at accounting for the health status of the respondents. There are significant paths but the variance accounted for in absolute and relative terms is small. Physical Health does a much better job of accounting for general mental health than it does for perceived health problems or physician determined illness. These findings suggest that the SF-36 may not discriminate well between healthy and non-healthy groups and that objective measures of health status may be required in conjunction with the use of the SF-36.