Lymphoma is one of the most common malignant tumours occurring in dogs. Since there is a constant need for new, more comprehensive laboratory diagnostic tools which permit the precise determination of many tumour-related factors we decided to verify whether the use of microarray analysis could be helpful in classifying lymphomas. The study was performed on samples collected from 7 dogs in which multicentric lymphoma was recognized. Among this group we were able to identify one sub-cluster of transcriptionally similar tumours, which completely differed in terms of the histopathological examination. Among them there were one diffuse large B cell lymphoma, one diffuse macronucleolated medium-sized cell lymphoma and one pleomorphic mixed small and large T-cell lymphoma. The lymphomas belonging to the sub-cluster differed from other analysed tumours in the expression of more than 100 genes of which only 18 were described earlier in regard to lymphomas.