This study attempted to develop, implement, and evaluate an experimental preschool education program based on Piaget's theory of cognitive development. A further goal was to examine Piaget's theoretical assumptions and postulated cognitive developmental trends for a 2-year period of the preoperational substage. A total of 48 3- to 5-year-olds participated in the project for the full 2-year period. Half the children attended the experimental Piagetian preschool and half attended a conventional preschool program. Evaluation measures used were the Peabody Picture Vocabulary Test, the RAVEN Coloured Progressive Matrices, and eight representative Piagetian tasks on seriation, classification, transitivity, conservation, measurement, and class inclusion. Normative longitudinal and cross-sectional analyses were used to examine data within the general area of the acquisition of cognitive abilities; within-stage intraindividual performance correspondences, developmental sequences in task performances, experimental/control group comparisons, and sex differences in task performance. The results suggested that although Piagetian theory provided a very workable and stimulating foundation for a preschool curriculum, program effects in this research were overshadowed by the large degree of individual variation in the rate and sequence of cognitive developmental acquisitions in the preoperational stage. (JMB)