Background: The aim of this study was to test the usefulness of the Cognitive and Language scales Bayley-III in the early assessment of cognitive and language functions in the context of an autism spectrum disorder (ASD) diagnosis. This paper focuses on the application of the Bayley-III and studies the predictive value of the test result in children with ASD with different levels of verbal ability. Method: A sample of 135 children (121 boys, 14 girls) with a confirmed ASD diagnosis at age 4 years were assessed with the Bayley-III before 42 months of age (m = 36.49, s = 4.46) and later with other rating scales of different psychological and psycholinguistic functions as part of a longitudinal study ["McCarthy Scales of Children's Abilities" (MSCA) (n = 48, 90% boys), "Kaufman Assessment Battery for Children" (K-ABC) (n = 38, 87% boys) or "Illinois Test of Psycholinguistic Abilities" (ITPA) (n = 44, 89% boys)]. Age assessment in months: MSCA (m = 48.80, s = 3.33), K-ABC (m = 51.80, s = 7.17) and ITPA (m = 54.48, s = 3.34). Results: Lower scores on the cognitive and language Bayley-III scales before 3.5 years of age predicted lower cognitive and oral language levels at 4 years of age. A significant correlation was found between the Cognitive Bayley-III Scale and the General Cognitive MSCA Scale, and with the Compound K-ABC Mental Processing. An association between the nonverbal cognitive level and oral language level acquired at 4 years of age was found. Conclusions: The Bayley-III is a useful instrument in cognitive and language assessment of ASD.