The study aim was to explore the relationship between a developmental assessment at preschool age and an intelligence quotient (IQ) assessment at school age. One hundred sixty-two children were assessed at 2.5 years with the Bayley Scales of Infant and Toddler Development--Third Edition (Bayley-III) and then at 6.5 years with the Wechsler Intelligence Scale for Children--Fourth Edition (WISC-IV). The Bayley-III Cognitive Index score was the Bayley entity that showed the highest correlation with WISC-IV Full-Scale IQ (FSIQ; r = 0.41). There was a significant difference between the individual WISC-IV FSIQ and the Bayley-III Cognitive Index scores. Analyses showed an average difference of -4 units and 95% limits of agreement of -18.5 to 26.4 units. A multivariate model identified the Bayley-III Cognitive Index score as the most important predictor for FSIQ and General Ability Index (GAI), respectively, in comparison with demographic factors. The model explained 24% of the total FSIQ variation and 26% of the GAI variation. It was concluded that the Bayley-III measurement was an insufficient predictor of later IQ.