Dysregulation of the hypothalamic-pituitary-adrenocortical (HPA) axis is believed to be a valid biological marker of stress. The present study explored the possibility that dysregulation of the HPA axis may be an underlying mechanism of pain severity among patients with acute low back pain (LBP), which may serve as a risk factor for pain to become chronic. The principal aims of the study were to investigate whether an association exists between HPA axis dysregulation and pain severity, and second, to identify any variable that might mediate that association. Saliva for cortisol awakening responses analysis was collected from 37 LBP patients by a salivette (a plastic tube with a cotton roll) 0 and 20 minutes upon awakening. The same procedure was repeated daily for two weeks. Primary care physicians and pain management clinics referred LBP participants to the study. Pain severity was considered as an outcome measure, whereas gender, age, cortisol levels, and cortisol variability were considered predictors. Analyses revealed that lower cortisol levels were associated with increased pain severity, (R2 = .012), F(1, 489) = 21.57, p <.000. Conversely, analyses revealed that greater cortisol variability over the two-week period was associated with increased pain severity, (R2 = .022), F(1, 489) = 24.07, p <.000. For each equation, we then entered stress levels, gender, and age into a stepwise regression to determine whether they would lessen the association between cortisol levels and variability and pain severity. However, none of these variables were associated with pain severity, and therefore, none were potential mediators. Notably, these findings suggest that cortisol levels and cortisol variability may influence the pain experience in a unique facet. Future research will need to determine to what extent this neuroendocrine mechanism influences the transition from the acute pain stage to a chronic one.