Many pediatric intensive care units use the COMFORT-Behavior scale (COMFORT-B) to assess pain in 0- to 3-year-old children. The objective of this study was to determine whether this scale is also valid for the assessment of pain in 0- to 3-year-old children with Down syndrome. These children often undergo cardiac or intestinal surgery early in life and therefore admission to a pediatric intensive care unit. Seventy-six patients with Down syndrome were included and 466 without Down syndrome. Pain was regularly assessed with the COMFORT-B scale and the pain Numeric Rating Scale (NRS). For either group, confirmatory factor analyses revealed a 1-factor model. Internal consistency between COMFORT-B items was good (Cronbach's alpha = 0.84-0.87). Cutoff values for the COMFORT-B set at 17 or higher discriminated between pain (NRS pain of 4 or higher) and no pain (NRS pain below 4) in both groups. We concluded that the COMFORT-B scale is also valid for 0- to 3-year-old children with Down syndrome. This makes it even more useful in the pediatric intensive care unit setting, doing away with the need to apply another instrument for those children younger than 3. (C) 2011 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved.