OBJECTIVE: In English paediatric practice, English law requires that parents and clinicians agree the ‘best interests’ of children and, if this is not possible, that the courts decide. Court intervention is rare and the concept of best interests is ambiguous. We report qualitative research exploring how the best interests standard operates in practice, particularly with decisions related to planned non-treatment. We discuss results in the light of accounts of best interests in the medical ethics literature. DESIGN: We conducted 39 qualitative interviews, exploring decision making in the paediatric intensive care unit, with doctors, nurses, clinical ethics committee members and parents whose children had a range of health outcomes. Interviews were audio-recorded and analysed thematically. RESULTS: Parents and clinicians indicated differences in their approaches to deciding the child’s best interests. These were reconciled when parents responded positively to clinicians’ efforts to help parents agree with the clinicians’ view of the child’s best interests. Notably, protracted disagreements about a child’s best interests in non-treatment decisions were resolved when parents’ views were affected by witnessing their child’s physical deterioration. Negotiation was the norm and clinicians believed avoiding the courts was desirable. CONCLUSIONS: Sensitivity to the long-term interests of parents of children with life-limiting conditions is defensible but must be exercised proportionately. Current approaches emphasise negotiation but offer few alternatives when decisions are at an impasse. In such situations, the instrumental role played by a child’s deterioration and avoidance of the courts risks giving insufficient weight to the child’s interests. New approaches to decision making are needed.