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Abstract Background Serum thymidine kinase 1 (sTK1) activity is closely correlated with DNA synthesis. Objectives Evaluate sTK1 activity as a biomarker for treatment response and early detection of relapse in dogs with lymphoma. Animals Ninety‐seven client‐owned dogs with naive or relapsed lymphoma and 23 healthy dogs. Methods Prospective study. Serum TK1 activity measured by refined ELISA‐based method (DiviTum assay, Biovica International) before treatment, at clinical response, and every 4 weeks until relapse or last follow‐up. Results Serum TK1 activity was ≤20 Du/L in 96% (22/23) of healthy dogs. Pretreatment sTK1 activity was >20 Du/L in 88% (85/97) dogs with lymphoma. At clinical response, sTK1 activity was significantly lower in dogs with complete (CR, n = 36) versus partial (PR, n = 29) response (P < .0001). Sensitivity (Se) and specificity (Sp) of sTK1 activity for detecting nonfully responders were 76% and 100%, respectively, with cutoff of 119.5 Du/L (AUC, 0.90; 95%‐CI, 0.81‐0.98; P < .0001). In dogs with CR, a 5‐fold increase in sTK1 activity at a 4‐week interval predicted relapse at the subsequent 4‐week assessment with a Se 50% and Sp 94% (AUC, 0.72; 95%‐CI, 0.55‐0.90; P = .02). An increase of sTK1 activity (>2.7‐fold value measured at clinical response) predicted relapse at subsequent 4‐week assessment with a Se 61% and Sp 88% (AUC, 0.79; 95%‐CI, 0.64‐0.95; P = .004). Conclusions and Clinical Importance Monitoring sTK1 activity could help to detect complete responders and early disease progression in dogs with lymphoma.