The article presents the concept of the lexicon for students learning Polish as a foreign language. The lexicon aims at providing intermediate and advanced students with appropriate information on the 'difficult areas of Polish' which are: complicated and rare inflection patterns, accommodation roles, principles of semantic combination and words similar in their phonetic form or having similar word building structure. The contents of the lexicon is based on 11 000 language errors excerpted from the written essays produced by intermediate and advanced learners of Polish as a foreign language. Each entry contains grammatical, semantic and pragmatic information. The entries are illustrated with examples of the incorrect usage of the language from the gathered material and with the corrected structures. The lexicon contains problematical, lexical, phrasal and morpheme entries.