Idiopathic pulmonary fibrosis (IPF) is a progressive fibroproliferative disease that results in increasing morbidity. To date there is only one licensed therapy for this condition and other agents are needed for this attritional disease. Efforts to study other agents have been obstructed by an increasing division of opinion about the most clinically meaningful end-point of phase III clinical trials to demonstrate efficacy. Many clinicians believe that an agent that impedes progression of the disease is more than acceptable and will encourage the pharmaceutical industry to further develop their IPF programmes. We have been impressed by the behavioural and biological similarities of cancer and IPF, and wondered if lessons could be learned about clinical trial design from lung cancer studies. Here, we set out our arguments that the similarities with cancer justify comparing the magnitude of therapeutic effects in clinical trials in nonsmall cell lung cancer with those in successful trials in IPF. We demonstrate that efficacy is of a similar magnitude in the two chronic lung diseases. We recommend that the demonstration of similar magnitudes of progression-free disease effect in IPF, using appropriate indices, should be considered as clinically meaningful benefit in future phase III clinical trials of novel therapies.