Background: Sex differences are widely reported in clinical psychology but are rarely examined in interventions. Method: This mixed-method explorative study examined sex differences in 13 mothers and 10 fathers of children in the off-therapy phase of acute lymphoblastic leukaemia. Parents underwent an expressive writing intervention using the guided written disclosure protocol (GWDP). Results: Mothers had more negative mood profiles than fathers but improved more during the intervention. Conclusion: Though preliminary, our findings highlight the importance of sex as a potential moderator of intervention and treatment outcome that could be of great clinical significance.