Negative affectivity (NA) has been shown to be strongly and consistently correlated to patient reported, subjective health indicators. Therefore, subjective health indicators may or may not give an accurate assessment of the individual’s true health status. NA can be expected to act as a general nuisance factor in health research, one that taps organically spurious variance. The pervasive influence of NA may confound the results and complicate the interpretation of health related quality of life instruments, such as the SF-36. OBJECTIVE: The purpose of this study was to evaluate the potential influence of NA on the MOS SF-36. METHODS: We used Structural Equation Modeling (SEM) techniques to evaluate the SF-36 and the impact of NA on the Mental and Physical Components in a sample taken from the 1990 National Survey of Functional Health Status (NHS). RESULTS: The percent of shared variance of the Physical Health and Mental Health indicators and the Physical and Mental Health factors combined are physical function (.0603), role physical (.0817), bodily pain (.0720), health perception (.0600), role emotional (.0486), vitality (.0756), general mental health (.5207) and social function (.0811). General mental health (GMH) indicators are virtually identical to NA indicators. The percent of shared variance of the NA/GMH factor and Physical Health and Mental Health is .4422 and .9781 respectively. CONCLUSIONS: The present results suggest that about 47 percent of Physical Health and 98 percent of Mental Health is due to the influence of NA/GMH. This may account for the lack of discriminative ability, shown in previous studies, of the Mental Health factor and Mental Component Scale Score.