Backgound: IL-13 is hypothesized to be a critical mediator in the development and maintenance of the asthma phenotype. Aim: To assess clinical activity and safety profile of tralokinumab (TK; CAT-354), a human IgG4 monoclonal antibody that specifically neutralizes IL-13. Methods: After a 2-week run-in, 194 subjects (52% atopic) with uncontrolled moderate/severe asthma despite standard controller treatment (ACQ-6 ≥1.5 & ≥1 exacerbation in last year) received SC TK (150, 300, or 600 mg) or placebo (PBO) every other week (7 doses) in addition to continued controller treatment. The primary endpoint was change in mean ACQ-6 score at week 13 (combined TK groups vs PBO); secondary endpoints included prebronchodilator lung function and rescue β 2 -agonist use. Results: At baseline mean (SD) age was 47 yr (10.8), ACQ-6 score was 2.7 (0.6), prebronchodilator FEV 1 was 2.0 L (0.6). At week 13 mean (SD) ACQ change was -0.76 (1.0) TK vs -0.61 (0.9) PBO ( P =0.375). Increases from baseline in FEV 1 and FVC (table) together with reduction in mean (SD) β 2 -agonist use (puffs/d) of -0.68 (1.5) TK vs -0.10 (1.5) PBO ( P =0.02) were observed following TK. No serious adverse events were considered drug-related by investigators. Conclusion: Adding tralokinumab to existing controllers was associated with an increase in FEV 1 but no improvement in ACQ score.