Canine testicular tumours are one of the most frequent tumours of the dog genital system and among the most frequent tumours of the dogs generally. Most tumours are diagnosed at a later stage when enlargement of the testicles is already present wherein the only and the best treatment method used is castration, after which a final diagnosis can be set up based on histopathological analysis of affected testicle. In this research, histopathological analysis was conducted on the archive samples of the most frequent canine testicular tumours (9 samples) and on one sample without the presence of a tumour changes, also the histopathological classification of the tumours has been conducted. The same samples were exposed to DNA analysis using flow cytomety to determine the proportion of tumor cells in certain stages of the cell cycle. With conducted comparative analysis of the results of histopathological evaluation and flow cytometry we tried to determine wheter there is a correlation between the histologically classified types of tumors and results of flow cytometry or whether changes in the genetic material (aneuploidy, an increased number of mitosis) or physical properties (size, shape, granularity) of tumor cells were associated with histological type of tumor, which is very common in testicular tumors of men. With implementation of both studies, it was found that a histopathological evealuation is an objective diagnostic method for distinguishing certain types of canine testicular tumors. The obtained results of flow cytometry, although they were not sufficient and precise enough for making a definitive conclusion, on the characteristics of the studied neoplastic changes show that with usage of the method of flow cytometry on the samples of canine testicles it is possible to get some new information about the properties of the cells (the number of chromosomes, the percentage of cells in each stage of the cell cycle). In order to flow cytometry and its results can be objectively analyzed and used in the diagnosis of testicular neoplasia in dogs it is necessary to optimize and standardize the method and detailed set of diagnostic criteria. Such criteria should then be compared with the results of histopathological analysis of the 35 tumours for what is necessary to conduct extensive research on a much larger number of samples of unmodified and neoplasticaly modified testicles. Flow cytometry provides many possibilities and leaves much space for improvement in the diagnosis of canine testicular tumours, especially if it was combined with well-established methods such as histopathological evaluation of testicular tumours.