Objective To examine the role of cortisol in posttraumatic stress symptomatology among mothers of children newly diagnosed with cancer. Methods Mothers (N = 27) completed standardized measures of posttraumatic stress symptoms and provided salivary cortisol samples at the time of their child's cancer diagnosis and then monthly for 1 year. Results Random effects regression analyses of 351 person-by-time observations revealed that high levels of cortisol were associated with higher levels of posttraumatic stress symptoms (B =. 12, p <. 02). The mothers who exhibited higher cortisol levels at the time of their child's diagnosis showed statistically significant declines in symptomatology from diagnosis to 12 months postdiagnosis (B =. 97, p <. 0001) compared to mothers who exhibited lower cortisol levels at diagnosis (B =. 003, p <. 05). Conclusion s These findings offer some suggestions into possible neurobiological processes underlying posttraumatic stress symptoms and directions for future research and clinical intervention. © 2010 The Author.