Utterances produced during spontaneous play activities by 180 Cantonese-speaking children, ranging in age from 3 to 5 years, were analysed with the focus on declaratives. Syntactic development was gauged in terms of changes in the mean length of utterance, sentence type and structure, syntactic complexity, and verb pattern, and age-related develpments in these were found. Significant sex differences were found in syntactic development, with girls outperforming boys in mean utterance length, some sentence types and structures, and syntactic complexity, with a significant age by sex interaction in the group of 4-year-olds. The period between age 3 and age 4 was identified as critical for syntactic development, as many linguistic changes occurred in this time. Growth in the ability to use compound sentences was found to be the most significant contributor to increased mean length of utterance. Biological, psychological, and sociocontextual factors influencing these sex differences in language performance are explored and discussed. The generality of the educational implications is discussed.