Activated protein C (APC) resistance is commonly tested in hypercoagulation evaluations, and argatroban is an anticoagulant therapy used in hypercoagulable patients. The effect of argatroban on APC resistance testing is unknown. We studied 100 ex vivo specimens from 44 patients treated with argatroban. Argatroban increased the APC resistance ratio in all but 1 patient. The effect was seen even in the specimens containing subtherapeutic argatroban levels. For patients without APC resistance, the mean APC resistance ratio significantly increased from 2.43 without argatroban to 3.10 with argatroban. For patients with APC resistance due to factor V Leiden, argatroban significantly increased the mean ratio from 1.73 to 2.13, falsely raising it into the normal range (>2). Argatroban increases the APC resistance ratio, which can mask a diagnosis of APC resistance and, therefore, factor V Leiden. Clinicians should be advised that APC resistance testing for patients receiving argatroban is not valid.