Anti-citrullinated protein antibodies (ACPA) are highly specific for rheumatoid arthritis and according to the ACR-EULAR 2010 guidelines represent an important serological marker for the diagnosis of the disease. Patients with rheumatoid arthritis who are positive for ACPA display more severe bone erosion and have an overall poorer prognosis in progression of the disease compared to ACPA negative patients. For a long time it was unknown how ACPA exactly affect bone homeostasis. In this article, recent findings about the mechanisms by which ACPA contribute to bone loss are discussed.