What is known and objectiveKaposi's sarcoma (KS) is a malignant neoplasm caused by HHV-8, a pathogen that leads to endothelial cell transformation when host defences are weakened. Case descriptionHere we report the first case of KS during treatment with abatacept, a biologic agent targeting T-cell costimulation. The patient was a 64-year-old female with rheumatoid arthritis who developed multiple firm, purple-reddish nodules on the dorsal aspect of the right hand. Histological examination confirmed KS. What is new and conclusionAlthough a direct causal relationship between KS development and abatacept treatment cannot be proved, we hypothesize a role for costimulation blockade.