This phase 2, randomised, double-blind, placebo-controlled trial evaluated the efficacy and safety of lebrikizumab, an interleukin-13 monoclonal antibody, alone or with background pirfenidone therapy, in patients with idiopathic pulmonary fibrosis (IPF).Patients with IPF aged ≥40 years with % predicted forced vital capacity (%FVC) 40%-100% and diffusing capacity for carbon monoxide 25%-90% and who were treatment-naive (Cohort A) or receiving pirfenidone (2403 mg·day-1; Cohort B) were randomised 1:1 to receive lebrikizumab 250 mg or placebo subcutaneously every 4 weeks. The primary endpoint was annualised rate of %FVC decline over 52 weeks.In Cohort A, 154 patients were randomised to receive lebrikizumab (n=78) or placebo (n=76). In Cohort B, 351 patients receiving pirfenidone were randomised to receive lebrikizumab (n=174) or placebo (n=177). Baseline demographics were balanced across treatment arms in both cohorts. The primary endpoint (annualised rate of %FVC decline) was not met in Cohort A (lebrikizumab versus placebo, -5.2% versus -6.2%; p=0.456) or Cohort B (lebrikizumab versus placebo, -5.5% versus -6.0%; p=0.557). In Cohort B, a non-statistically significant imbalance in mortality favouring combination therapy was observed (hazard ratio, 0.42 [95% CI, 0.17-1.04]). Pharmacodynamic biomarkers indicated lebrikizumab activity. The safety profile was consistent with that in previous studies of lebrikizumab and pirfenidone as monotherapies.Lebrikizumab alone or with pirfenidone was not associated with reduced %FVC decline over 52 weeks despite evidence of pharmacodynamic activity. Lebrikizumab was well tolerated with a favourable safety profile. These findings suggest that blocking IL-13 may not be sufficient to achieve a lung function benefit in patients with IPF.