The aim of this study was to analyze the most common diseases and genetic defects that occur during the lifetime of giant dog breeds, to determine the average lifespan and the cause of death/euthanasia. Data were obtained through a survey and concerned the health of 241 individuals of giant dog breeds held in the Czech Republic. Evaluated items involved an average lifespan, an average lifespan per gender, cause of death, reasons for euthanasia, cause of mortality (especially in selected Mastiff type breeds), life expectancy per breed and incidence of diseases among giant breeds during the lifetime. The average lifespan in giant breed dogs was found to be 7.60 years. A significant difference (P ˂ 0.05) was found between life expectancies in males and females, with female dogs reaching 1.42 year higher age (8.10 years) than males (6.68 years). The most common cause of spontaneous death among giant breeds was gastric dilatation and torsion (28% of dogs) and for euthanasia osteosarcoma (38% of dogs). The cause of mortality especially in selected Mastiff type breeds of dogs was gastric dilatation and torsion (30% of dogs). This is the first broad analytical study concerning this topic published in the Czech Republic.