This paper provides a detailed description of Mean Length of Utterance (MLU) including a research experiment with a view to getting a comparative result on counting MLUm and MLUw - two specialized versions of MLU. In this experiment the data of a normally developing child and a child with language disorder have been taken from CHILDES database. The result which is relevant to some other previous studies shows that there exists a strong correlation between MLUm and MLUw. Thus, as a means of calculating MLU, MLUw could be preferred because it is easier, less technical in terms of grammatical complexity as well as a language or dialect neutral procedure.Key words: Mean Length of Utterance, MLUm, MLUw, CHILDES, SLI, CLAN Dhaka University Journal of Linguistics Vol.1(1) 2008, 167-182