- To better understand differences between Bayley Scales 3rd edition (Bayley III) Cognitive Scale and Bayley Scales 2nd edition (Bayley II) Mental Developmental Index (MDI) in 18-22-month-old children born term and preterm and to create a conversion algorithm using Bayley II MDI to calculate Bayley III Cognitive score.This study included 51 term and 26 preterm children between 18 and 22 months, ages adjusted for prematurity. Children's scores on Bayley II MDI and Bayley III Cognitive Scale were compared using t-tests. The items from Bayley II MDI were used to calculate a score for the Bayley III Cognitive Scale. ANCOVA was used to create a conversion scale.Bayley III Cognitive scores were significantly higher than Bayley II MDI scores for term and preterm toddlers combined and separately (p < .0001). A conversion formula to convert Bayley II MDI to a Bayley III Cognitive score was calculated.Term and preterm children had similarly elevated scores on the Bayley III calculated Cognitive score compared to the previous Bayley II MDI score. The use of a conversion algorithm may be helpful in studies that used both Bayley editions to get comparable outcome measurements within a clinical or research paradigm.© 2011 The Author(s)/Acta Paediatrica © 2011 Foundation Acta Paediatrica.