Objective: To evaluate the risk of and predictors of enduring and late-onset posttraumatic stress disorder (PTSD) among mothers of children diagnosed with type I diabetes and cancer. Method: Mothers (N = 99) of children diagnosed with cancer or diabetes for at least 12 months completed a structured clinical interview for PTSD and self-report measures of PTSD, depression, anxiety, and stressful life events. Results: There was no significant difference in the rate of PTSD between the two groups. Overall, fewer mothers (7%) met criteria for PTSD on the structured clinical interview than those on a self-report measure of PTSD (17%). Mothers who reported more depressive symptoms, anxiety, and stressful life events tended to report significantly more PTSD symptoms. Conclusions: The findings extend prior research regarding the prevalence rate and predictors of PTSD and PTSD symptoms in pediatric populations. It is recommended that clinicians exercise caution when interpreting prevalence rates for PTSD that are derived from self-report measures. © The Author 2006. Published by Oxford University Press on behalf of the Society of Pediatric Psychology.