Progress in life expectancy and the growing number of people living to old age intensify the phenomenon of multi-morbidity, defined as the coexistence of several chronic diseases. By exploiting all the medical information in death certificates, the multiple causes of death (MCoD) approach serves to investigate complex pathological processes that lead eventually to death. This is the first MCoD analysis for Poland and its objective is twofold: to examine the quality of information on contributing causes of death, in particular in the regional dimension, and to assess the scale of multi-morbidity involving conditions that are becoming more and more frequent in ageing populations. The analysis is carried out for all deaths that took place in Poland in 2013. The results show that medical doctors issuing death certificates often define contributing causes of death, but a large part of this information includes unknown or ill-defined conditions. Several conditions favour the certification of well-defined contributing causes: when death occurs in hospital, or is due to underlying causes other than cardiovascular, the number of contributing conditions is higher. Important regional differences are observed in this regard. The analysis highlights the importance of diseases that are rarely certified as the underlying causes, but often contribute to mortal conditions, such as diseases of the blood and the blood-forming organs, diseases of the skin and subcutaneous tissue, diseases of the genitourinary system or mental and behavioural disorders.