The objective of this study was to determine whether children with nonsyndromic craniosynostosis and plagiocephaly without synostosis demonstrated cognitive and psychomotor delays when compared with a standardized population sample. This was the initial assessment of a larger prospective study, which involved 21 subjects with nonsyndromic craniosynostosis (mean age, 10.9 months) and 42 subjects with plagiocephaly without synostosis (mean age, 8.4 months). Each child was assessed using the Bayley Scales of Infant Development-II (BSID-II) for cognitive and psychomotor development before therapeutic intervention (surgery for craniosynostosis and molding-helmet therapy for plagiocephaly without synostosis). The distribution of the scores was divided into four groups: accelerated, normal, mild delay, and significant delay. The distributions of the mental developmental index (MDI) and the psychomotor developmental index (PDI) were then compared with a standardized Bayley's age-matched population, using Fisher's exact chi-square test. Within the craniosynostosis group, the PDI scores were significantly different from the standardized distribution (p < 0.001). With regard to the PDI scores, 0 percent of the subjects in the craniosynostosis group were accelerated, 43 percent were normal, 48 percent had mild delay, and 9 percent had significant delay. In contrast, the MDI scores were not statistically different (p = 0.08). Within the group with plagiocephaly without synostosis, both the PDI and MDI scores were significantly different from the normal curve distribution (p < 0.001). With regard to the PDI scores, 0 percent of the subjects in the group with plagiocephaly without synostosis were accelerated, 67 percent were normal, 20 percent had mild delay, and 13 percent had significant delay. With regard to the MDI scores, 0 percent of the subjects in this group were accelerated, 83 percent were normal, 8 percent had mild delay and 9 percent had significant delay. This study indicates that before any intervention, subjects with single-suture syndromic craniosynostosis and plagiocephaly without synostosis demonstrate delays in cognitive and psychomotor development. Continued postintervention assessments are needed to determine whether these developmental delays can be ameliorated with treatment.