Purpose: We examined the properties of mean length of utterance (MLU) in Czech, a morphologically complex Slavic language. We compared the scores of MLU calculated in different units and based on different sample lengths and assessed its validity against another transcript and test-based measures. Method: One hundred nine children were recorded during free-play at 2;6 and 3;11 (years;months). We compared MLU in syllables, morphemes, and words (MLU[subscript w]) in transcripts of different lengths (50, 75, 100, and all available utterances). For evaluating the validity of MLU, we also calculated Index of Productive Syntax (IPSyn) and number of different words (NDW) and used results of receptive vocabulary and grammar comprehension tests. Results: The different MLU measures based on different sample lengths correlated closely with MLU in transcripts of all utterances (all "rs" > 0.87). We found mostly strong correlations between MLU, IPSyn, and NDW at both time points and weak or moderate correlations between MLU and grammar and vocabulary. Regression models showed the significant unique effect of MLUw at 2;6 for MLUw ([beta] = 0.29) and grammar ([beta] = 0.33) at 3;11 and vocabulary ([beta] = 0.27) at 3;7. Conclusion: MLU[subscript w] based on all utterances was confirmed as a valid measure of early language skills in Czech, as it is stable in time and shows concurrent and predictive relations with other transcript-based and test-based measures.