Background and objective: It is currently recommended that patients avoid large meals prior to their lung function tests. The aim of this study is to determine whether this recommendation is necessary in clinical practice. Methods: A randomized controlled cross-over trial was conducted. Subjects performed lung function tests (spirometry, measurement of lung volumes and gas transfer) prior to, directly following and 2 h after consuming a large breakfast. On the control arm, subjects performed the same lung function tests while fasting for the duration of the morning. The study subjects comprised 12 healthy subjects, 10 COPD patients and 10 patients with interstitial lung disease. Results: There were no significant differences between measurements on the meal and control days for FEV1, FVC, TLC or DLCO. There were no significant changes with time in any of these parameters over the course of either the meal or control morning. Conclusions: Common measures of lung function are not affected by the prior consumption of a large meal and it is unnecessary to advise patients to avoid a large meal prior to lung function measurement.