OBJECTIVE: The primary objective of the MEGA-J study (UMIN-CTR number: UMIN000057308) was to assess the safety of CT-P13, an infliximab biosimilar, in Japanese patients with Crohn's disease (CD), ulcerative colitis (UC), and/or rheumatoid arthritis (RA) after switching from reference infliximab. METHODS: Data were collected over 5 years, following initiation of CT-P13 treatment within routine clinical practice. Interim findings are reported (cut-off: last patient's Year 2 visit). The primary endpoint was the incidence of uncommon adverse drug reactions (ADRs), including tuberculosis and serious infections. RESULTS: Overall, 220 patients were enrolled (123 CD; 74 UC; 23 RA). Forty-eight (39.0%), 37 (50.0%), and 3 (13.0%) patients reported >/=1 uncommon ADR in the CD, UC, and RA groups, respectively. The majority (94.3%) were unrelated to CT-P13. No cases of tuberculosis and one unrelated case of serious infection were reported. Nineteen (8.6%) patients discontinued treatment for reasons related to CT-P13. CONCLUSIONS: Overall, CT-P13 was well tolerated, demonstrating the safety of long-term treatment in real-world practice.