Life-threatening illnesses in children have a significant impact on the lives of their brothers and sisters. Consequently, special attention must be paid to the specific needs of these siblings to help them cope with their situations. To address this issue, we developed an inventory of the needs of the adolescent siblings of severely ill children, the Inventaire des Besoins de la Fratrie d'Enfants Malades Severement (IBesFEMS) [Needs Inventory for Siblings of Critically Ill Children]. The present article describes a preliminary validation study of this new instrument. In a prospective cohort study, the 48-item instrument was administered via a website or paper to 58 siblings. Our study revealed that the measure has an estimated internal consistency of 0.96 and a temporal stability intraclass correlation coefficient ([i]ICC[/i]) of 0.86 ([i]p[/i] less than 0.01). Its convergence validity is also satisfactory. Our findings suggest that the IBesFEMS is highly relevant for pediatric palliative care clinicians and researchers. Future studies should investigate its factorial structure and predictive validities.