On the basis of the statutory code of physicians' duties in Poland, the authors discuss a new category of professional error, namely "error in expert assessment." Expert assessment comprises: Issue of certificates stating temporary inability to work (L-4 forms); Issue of certificates allocating patients to appropriate disability categories; Issue of certificates entitling patients to special services; Issue of certificates for legal purposes According to regulation any physician can be required to provide an expert assessment, because institutions requesting such an opinion refer to persons or institutions so as to obtain special information. In certain cases physicians may request to be released from this obligation. Nevertheless, if they accept it and then pass an erroneous opinion they may be liable to a charge of malpractice. The authors have pointed out that errors in expert assessment usually result from: Inadequate professional knowledge; Examination of the records with no clinical examination; Failure to take account of information included in the patient's file; Illogical or unjustified conclusions about the cause and result relationship. The most common errors in the formulation of expert assessments are: "Overdiagnosis" of the results of an accident; Unjustified statements that death has resulted from an accident or from bad working conditions in cases of death from natural causes; Incorrect reconstruction of the events leading up to an accident or of the mechanism of injuries.(ABSTRACT TRUNCATED AT 250 WORDS)