In a previous study, the authors found that structured writing about stressful events improved symptomatology in 112 patients with rheumatoid arthritis and asthma relative to patients who did not write (J. Smyth, A. Stone, A. Hurewitz, & A. Kaell, 1999). However, little is currently known about the pathways from the intervention to alterations in outcomes. In addition to measuring symptom outcomes after the intervention in the previous study, the authors monitored perceived stress, quality of sleep, affect, substance use, and medication use on a momentary basis for the 7 days prior to writing, during the 3 intervention days, and for the 14 days following the intervention (N = 105). These variables were tested in a secondary data analysis to determine whether they mediated the effects observed in the J. Smyth, A. Stone, et al. study. No evidence was found supporting mediation, and the mechanism underlying structured writing about stressful events remains unknown.