A 24-week multicentre study aimed to compare the performance of the biosimilar SB4 to that of its reference drug, etanercept, in a total of 596 patients with moderate to severe rheumatoid arthritis also receiving methotrexate. The results confirm that SB4 and etanercept have equivalent clinical efficacy, and that the two agents have generally comparable safety and pharmacokinetics. SB4 showed lower immunogenicity than etanercept, thought to be related to the absence of L-arginine in SB4, but this difference does not influence its biosimilar status.