Background <p>Lecanemab is a disease-modifying immunotherapy for Alzheimer’s disease (AD) and has been increasingly prescribed to the patients in the world. It has a definite therapeutic effect in slowing cognitive decline in AD patients, but its adverse drug reactions should not be ignored.</p> Case presentation <p>We reported an 82-year-old Chinese AD patient who developed recurrent D-dimer elevation after lecanemab infusions. His D-dimer levels were positively correlated with activated partial thromboplastin time, C-reactive protein, and neutrophil percentage, but negatively correlated with platelet count.</p> Conclusions <p>Elevated D-dimer levels may be associated with the enhanced phagocytic response to vascular amyloid β upon lecanemab treatment. Our report expands knowledge on lecanemab’s adverse drug reactions, suggesting that D-dimer levels and potential thromboembolic events should be monitored during treatment.</p> Clinical trial number <p>not applicable.</p>