The study examines the parameter of Mean Length of Utterance (MLU), measured both in morphemes (MLUm) and words (MLUw), in early language development in the case of two English children matched for age. The MLU scores of a normally developing child were compared to the MLU results of a language-impaired child in a longitudinal study. Moreover, the reliability of the MLU index measured in words was also tested in both children. The MLU analysis was based on the CHILDES database and CLAN programme, where the transcripts of spontaneous speech samples are used to calculate basic language parameters at different age-points. The findings of this study indicate that despite the expected delay, the language-impaired child followed a similar route of language development as the control child. However, significant differences between MLUw and MLUm confirmed that the parameters performed two different linguistic analyses.