The paper describes a morphological analyser forEstonian and how using a text corpus influenced theprocess of creating it and the resulting programitself. The influence is not limited to the lexicononly, but is also noticeable in the resulting algorithm andimplementation too. When work on the analyser began,there were no computational treatment of Estonianderivatives and compounds. After some cycles ofdevelopment and testing on the corpus, we came up withan acceptable algorithm for their treatment. Both themorphological analyser and the speller based on ithave been successfully marketed.