Mean length of utterance (MLU) in morphemes was examined as a predictor of the grammatical complexity of natural language corpora of normal preschoolers and of children and adolescents with delayed language, Fragile X syndrome, Down syndrome, and autism. The Index of Productive Syntax (IPSyn) served as the measure of syntactic and morphological proficiency. For all groups, a strong curvilinear association between measures was found across the MLU range from 1.0 to about 4.5. Correlations were weaker when MLU exceeded 3.0 than during earlier stages of language development, however, confirming previous suggestions that MLU becomes less closely associated with grammatical development as linguistic proficiency increases. For the language-disordered groups, moreover, the curves relating the two measures differed from the curves for the normal preschoolers because MLU frequently overestimated actual IPSyn scores. The results are discussed with respect to the use of MLU in conjunction with other measures of syntactic complexity in the study of atypical language development.