Background: Thrombosis and arterial thromboembolism are frequent complications of feline cardiomyopathy, especially when associated with left atrial enlargement. Markers of activated coagulation may be used to evaluate the coagulation status of cats with hypertrophic cardiomyopathy (HCM) in relation to left atrial size. Objectives: The objective of this study was to compare plasma concentrations of thrombin-antithrombin complex (TAT), D-dimer, and fibrin degradation products (FDP) between clinically healthy cats and cats with HCM. Prothrombin time (PT), activated partial thromboplastin time (aPTT), and antithrombin activity were also compared and the association between left atrial (LA) size and coagulation results in cats with HCM was evaluated. Methods: Blood samples from 19 clinically healthy cats and 20 cats with HCM were obtained. All cats with HCM were asymptomatic and had no signs of heart failure. LA diameter and LA to proximal aortic (Ao) diameter ratio (LA:Ao) were determined by echocardiography. Results: Reference intervals for D-dimer and TAT concentrations in plasma of healthy cats were established as 0.09–0.32 μg/mL and 2.0–20.0 μg/L, respectively. TAT, D-dimer, and FDP concentrations were increased in 5, 3, and 2 cats with HCM, respectively. TAT and D-dimer concentrations, and PT and aPTT were not significantly different between groups. Antithrombin activity was significantly decreased in cats with HCM (P= .03) despite marked range overlap. LA and LA:Ao were not correlated with coagulation results. Conclusions: Laboratory evidence of hypercoagulability was found in 45% of cats with HCM. Left atrial size was not associated with laboratory evidence of hypercoagulability. Association between coagulation markers and risk of thrombosis has yet to be evaluated in cats with HCM.