In a study on cognitive style, reflectivity-impulsivity in response to task uncertainty was investigaged. At 44 months of age, 23 boys and 25 girls were tested on a matching-figures test. Correlations were made between number of errors, response time, intelligence, and sex. Results indicated that there were significant sex differences in preschool children's cognitive style. Boys' errors were significantly correlated with response time but not with IQ. Girls' errors and response time were significantly correlated with intelligence, but their errors and response time were not significantly correlated with each other. Similar sex differences have been found in studies using older children. Findings indicate that data by sex should not be pooled and that a more careful exploration of sex differences in cognitive style should be made. (MS)