The notion of language dominance is often defined in terms of proficiency. We distinguish dominance, as a property of the bilingual mind and a concept of language knowledge, from proficiency, as a concept of language use. We discuss ways in which language dominance may be assessed, with a focus on measures of mean length of utterance (MLU). Comparison of MLU in the child's 2 languages is subject to questions of comparability across languages. Using the Hong Kong Bilingual corpus of Cantonese-English children's development, we show how MLU differentials can be a viable measure of dominance that captures asymmetrical development where there is an imbalance between the child's 2 languages. The directionality of syntactic transfer goes primarily from the language with higher MLU value to the language with lower MLU value, and the MLU differential matches the pervasiveness of transfer effects, as in the case of null objects discussed here: The greater the differential, the more frequent the occurrence of null objects. Cantonese-dominant children with a larger MLU differential use null objects more frequently than those with a lower MLU differential. In our case studies, MLU differentials also matched with language preferences and silent periods but did not predict the directionality of code-mixing. (Contains 12 footnotes, 3 tables and 3 figures.)