Background and Objectives: Research examining the role of cortisol in posttraumatic stress disorder (PTSD) has largely been cross-sectional studies and few studies have examined cortisol in relation to specific symptom clusters. Examining cortisol in relation to specific PTSD symptom clusters could aid in identifying candidates for symptom-specific treatments. Hence, cortisol was examined in relation to specific PTSD symptom clusters including reexperiencing, avoidance, numbing, and hyperarousal symptoms. Design: A repeated-measures longitudinal design was utilized to predict PTSD symptom clusters. Methods: Mothers of children (N = 27) diagnosed with cancer completed a measure of PTSD, and they provided salivary cortisol samples at the time of their child's diagnosis as well as monthly for the following 12 months. Results: Multi-level modeling analyses revealed that higher cortisol levels were significantly related to higher levels of numbing symptoms. Although numbing symptoms declined as cortisol levels declined across 12 months postcancer diagnosis, mothers with higher cortisol levels still reported more numbing symptoms. Reexperiencing, avoidance, and hyperarousal symptoms were not found to be related to cortisol level across time. Conclusions: The findings offer support for the role of cortisol in the manifestation of numbing symptoms. Further research is recommended with other trauma groups to maximize generalizations.