Nontuberculous mycobacterial (NTM) disease and tuberculosis (TB) develop during antitumour necrosis factor (TNF)-a therapy. We compared clinical characteristics and outcomes between the two diseases.A total of 1165 patients were screened for TB and treated with TNF-a antagonists from July 2004 to July 2013 for the following conditions: inflammatory bowel disease (n5422), rheumatoid arthritis (n5320), and ankylosing spondylitis (n5389).TB and NTM disease were diagnosed at baseline screening in four and three patients, respectively, and developed during anti-TNF-a therapy in 19 and six patients, respectively. The incidence rate of TB and NTM disease was 747.7 per 100 000 and 238.2 per 100 000 person-years, respectively. Patients with NTM disease were older, with a greater proportion of females. All cases of NTM disease involved the lung, with rheumatoid arthritis (83.3%) being the most frequent underlying disease. The most common radiological feature was consolidation in NTM disease, and honeycombing was present in two rheumatoid arthritis patients with NTM disease. The most common pathogen was Mycobacterium intracellulare (n53) followed by Mycobacterium avium (n52). Both the NTM and TB group showed favourable outcomes.The clinical characteristics differed between NTM disease and TB that developed on anti-TNF-a agents, but clinical outcomes were favourable in both diseases. @ERSpublications Clinical characteristics differ between nontuberculous mycobacterial disease and TB that develop on anti-TNF-a agents