Background: The definition of "inability to work" and principles of medical certification of disabilities, determining the entitlement to an ill-health pension provision are regulated by law. However, the term „"inability to work" is not precisely defined and the standards of its assessment are not accurately formulated. This refers not only to the term "loosing ability to work to considerable degree", but also to the criteria of occupational qualifications. Objectives: The aim of this study was to identify the most common problems and mistakes in the medical certification that entitles to disability pension provision. Materials and Methods: The study, based on medical consultations provided for the president of the Social Insurance Company (ZUS) in the years 2005-2007, was carried out in the Out-Patient Clinic of Occupational Diseases, Nofer Institute of Occupational Medicine, Łódź. In total, 293 medical cases were  analyzed, of which 61 concerned consequences of occupational diseases, 44 - accidents at work and 188 - idiopathic diseases. Results: Men formed the majorityof the study group (66%) and the mean age of the examined individuals was 49. The predominant causes of medical certification of patient's inability to work, both in men and female, were musculoskeletal disorders, cardio-vascular diseases, diabetes and mental health problems. Significant discrepancies in the medical statements on inability to work were observed between ZUS and the Nofer Institute of Occupational Medicine. They concerned the evaluation of people's inability to work in general, as well as the degree (partial or complete) of lost capability for work. A consensus of medical statements was found in 153 cases, and differences in other 140 (47.8%) cases. Most differences were found in the evaluation of inability to work caused by consequences of occupational diseases. Conclusions: Lack of certification guidelines, excessive preference of medical criteria used for defining body fitness disturbances and insufficient consideration of occupational and economic aspects of certified inability to work in terms of assuming work in future are the main reasons for difficulties in inability to work certification. The scheme of certification procedure was also suggested. Med Pr 2010;61(1):23-33