The goal of the thesis was to ascertain what diseases occur most frequently among medium-sized dog breeds and what average life expectancy they have. Furthermore to find out what factors significantly influence the life expectancy. A questionnaire was created for dog breeders in order to specify scientific hypotheses. The basic assumption was that the average life expectancy of medium-sized dog breeds is 12 years. Further assumptions were that the average life span is not affected by sex, breeding management, castration and potential childbirth. In total, the questionnaires gathered information about 363 specimens (246 bitches and 117 dogs) of 61 breeds. Ten of the breeds (of 10 or more specimens) were used for further analysis. The average life expectancy of all the animals was 11.7 years. There were no differences between dogs and bitches as well as between individual breeds. Feeding and keeping the dogs outside or inside also had no influence on the life expectancy. On the other hand, it was influenced by keeping the animals in a pack as opposed to alone. Almost two thirds of the dogs had suffered no diseases during their lives and a disease occurred only as a cause of death with 43% of them. The most frequent diseases included tumours, hip dysplasia, ear canal inflammations, glaucoma, allergies, pyometra and arthrosis. Forty six percent of the dogs died or were put to death because of a disease. Most frequently the cause of death was a cancer, organ failure, stroke, pyometra, gastric dilatation and volvulus and epilepsy. Thirty eight percent of the dogs died or were put to death because of old age. Animals were euthanized in 42% of cases. Only 21% of the animals were castrated. However, castration proved to be a factor that influences the life expectancy. The influence of castration on males was not proved but castrated females lived longer than females that were not castrated. Seventy one percent of females gave birth at least once. These females lived longer that those who have never given birth. The number or the course of the litters has not been proved to influence the life expectancy of the bitches.