This study investigated the relationship between frequency of intrusive thoughts and susceptibility to reminiscent stimuli and consequent distress in parents of children with cancer and parents with healthy children. Cancer-related words embedded in the Stroop task (e.g., “chemo” printed in green ink), served as the reminiscent stimuli. Results indicated that frequency of intrusive thoughts reported 2 months before the experimental session was positively correlated with evoked thoughts and associated distress among parents with ill children. Intrusive thoughts predicted 11–17% of the variance in evoked thoughts and in elicited distress, whereas other symptoms of chronic stress did not predict evoked thoughts and elicited distress. Cognitive and affective task reactivity by parents of children with cancer were not accompanied by behavioral or physiological reactivity. Future research should examine the extent to which more acute and naturalistic intrusive thoughts elicit reactivity across cognitive, affective, behavioral, and physiological dimensions, and long-term physical and mental health effects associated with chronic intrusive thoughts.