The Czech Academic Corpus was created during the 1970s and 1980s at the Czech Language Institute under the supervision of Marie Tesitelova. The main motivation to build it (a total of 540 thousand word tokens) was to obtain the quantitative characteristics of contemporary Czech. The corpus is structurally annotated on two levels - the morphological level and the syntactical-analytical level. The original stochastic experiments in morphological tagging of Czech were performed using the corpus at the beginning of the 1990s. Given this, the corpus-based processing of Czech was launched. At the end of 1990s, work on the Prague Dependency Treebank had started (independently from the corpus) and its first edition was published in 2001. In considering future released versions of the treebank, we have decided to convert the corpus into the treebank-like format. This article focuses on the twenty-year history of the Czech Academic Corpus. Special attention is devoted to thus far unpublished facts about the corpus annotation. The conversion steps resulting in the first version of the Czech Academic Corpus are described in detail.