Background Developmental assessment is an important facility for early detection and intervention of developmental delay in children. Objective: to assess the performance of a sample of middle social class Egyptian infants and toddlers on Bayley Scales of Infant and Toddler Development-third edition (Bayley III), and to compare their cognitive, motor, and communication scores with that of the reference norms. Methods It was a cross-sectional pilot study, included 270 children aged 18-42 months. Mothers filled a questionnaire including questions about family socioeconomic background, perinatal history, and family history. Physical examination and growth assessment of children were performed. Developmental assessment of cognitive, language and motor skills was performed using the Bayley III scales and compared the American norm scores with the Egyptian mean scores. Results The mean cognitive, language and motor composite scores were 92.5+18.5, 91.76+ 15.6, and 95.67+18.9 respectively. All were lower than the American mean (100+ 15) with highly significant differences. About one-fourth of the enrolled Egyptian children had below-average composite scores according to the US cutoff point. The ranks of Egyptian children on the American versus the Egyptian percentile curves were significantly different. Conclusion Mean values of all assessed developmental domains of Egyptian children are within the norm-referenced average of Bayley III, but lower than the recorded American mean. Assessing Egyptian children according to the American norms may result in overestimating developmental delay. This pilot study raised the question about using different cutoff points suitable for the developmental trajectory of Egyptian children. Answering this question needs further studies on Bayley-III after cultural adaptation and standardization, using a larger, more diverse, and representative sample of the Egyptian population.