According to the Menzerath-Altmann law, the mean word length is greater in shorter clauses than in longer ones. In Czech, negation is mostly realized by adding the prefix ne- to the beginning of the word, which makes the word longer (and, consequently, it also increases the mean word length in the clause). Therefore, we predict that clauses in which the predicate is in the affirmative form are longer than ones with the negative predicate. We test the hypothesis on a sample of 59 pairs of affirmative and negative forms of the same verb from the Prague Dependency Treebank 3.0.