This article presents the results of a secondary analysis of data collected from a cross-sectional survey of 201 individuals with fibromyalgia syndrome (FMS). This study examines the influence of stress and family relationship functioning (differentiation of self) on symptoms severity among FMS patients. The primary analysis of the data (published elsewhere) revealed that perceived levels of stress were correlated with symptom severity, and that other variables related to family functioning moderated this relationship to a small degree. In the secondary analysis presented here, additional relationships among perceived stress, stressful life events, family relationship functioning, age of FMS onset, and symptom severity were examined. The results indicated that participants' levels of symptom severity could be predicted by their level of differentiation and appraisal of stressful life events occurring within a year prior to the age of onset. Consistent with the findings of the primary analyses, the level of perceived stress was the predictor variable that accounted for the greatest amount of variance associated with FMS symptoms. Limitations of the study and recommendations for future research are presented.