We examined whether a positive intervention (i.e. granting a wish) could promote positive psychological and physical changes (e.g. reduced nausea and pain) in seriously-ill children. Children and their parent were randomly assigned to a wish group (completed measures 2–3 days before the wish and 3 weeks later) or to a waiting-list control group (with an equivalent time-lag and receiving the wish after the assessment). Wish intervention significantly increased levels of positive emotions, satisfaction with life, personal strengths, and reduced rates of nausea compared with the control group. Mothers in the wish group also perceived positive changes in children’s benefit finding and quality of life.