Malignant lymphoma is a common lymphoproliferative neoplasm affecting dogs. Telomerase is an RNA-dependent DNA polymerase that adds telomeric repeats (TTAGGG)n to chromosome ends, compensating for losses which occur with each round of DNA replication. The objectives of this study were to determine whether lymph nodes, serum and buffy coat samples from dogs with malignant lymphoma (ML) have detectable telomerase activity using the telomere repeat amplification protocol (TRAP) assay. Furthermore, there was interest in the potential diagnostic and prognostic utility of the telomerase enzyme. An additional objective was to assess whether lymph nodes, liver, buffy coat and serum samples have detectable telomerase activity, as well as to identify other characteristics of 14 dogs with ML that might aid in predicting patient outcome. Variables such as the signalment of the animal, stage, grade and immunophenotype of the disease, as well as the presence of hypercalcemia were investigated for correlation with the disease-free interval and survival time. Normal tissue samples obtained from eleven young adult dogs undergoing terminal surgical procedures were obtained for determination of telomerase activity. Fourteen client-owned dogs with ML were referred to the Ontario Veterinary College, University of Guelph, for the staging and treatment of ML. The TRAP assay was used to quantify telomerase activity (total products generated [TPG units]) in the tissues from normal dogs and dogs with ML. Histopathology and immunohistochemistry were also performed on the lymph node samples from dogs with ML. Telomerase activity was present in eight of the 11 normal lymph node samples. Five of the 11 liver samples had telomerase activity, and one of the 11 buffy coat samples had minimal telomerase activity. None of the serum samples from the normal dogs had detectable telomerase activity. Nine of the 14 lymph nodes from dogs with ML had measurable telomerase activity. Three of the 14 dogs with ML had detectable telomerase activity in the buffy coat samples, and one of the 14 serum samples had measurable telomerase activity. Eight dogs were diagnosed with high grade tumours, while six dogs had intermediate grade tumours. No low grade tumours were identified. Immunohistochemistry was performed on ten of the 14 lymph nodes from dogs with ML. Six of the ten cases were identified as B-cell tumours, and four were of T-cell origin. A wide range in the telomerase activity was observed in lymph nodes of dogs with ML. However, normal lymph nodes and liver tissue may have levels of telomerase activity similar to the activity found in neoplastic lymph nodes. Thus, telomerase activity is not specific to tumour cells, and may overlap with that found in normal cells. Detectable levels of telomerase activity may be present in neoplastic serum and buffy coat samples, however, levels are generally low. No significant statistical associations were found between nodal telomerase activity and the age, sex, breed, stage, grade, immunophenotype, presence of hypercalcemia and buffy coat telomerase activity. These same variables were assessed for their ability to predict disease-free interval and survival time, and no significant statistical associations were identified. Although the determination of telomerase activity adds to the description of a neoplastic phenotype, it cannot be used as a sole diagnostic test. Therapeutic modalities directed toward the telomerase enzyme have been theorized, however, this may not be feasible because normal somatic tissues and renewal tissues possess variable levels of telomerase activity.