Twitch transdiaphragmatic pressure (Pdi,tw), measured following magnetic stimulation of the phrenic nerves, is used to assess diaphragm strength, contractility and fatigue. Although the effects of posture, lung volume and potentiation on Pdi,tw are well described, it is not known whether the degree of gastric filling affects the measurement. Pdi,tw was recorded in seven healthy volunteers on two occasions with antero-lateral magnetic stimulation of the phrenic nerves. On the first occasion, the subjects had fasted for at least 8 h, whilst on the second occasion, measurements were made after each subject had eaten a substantial meal sufficient to produce a feeling of satiation. Mean postprandial unpotentiated and potentiated Pdi,tw were significantly greater than corresponding fasting Pdi,tw in all seven volunteers (29.8 versus 25.7 cmH2O and 38.9 versus 34.4 cmH2O, respectively). This was due to a significantly increased gastric pressure component (1.10 versus 0.84 and 0.94 versus 0.78, respectively), and reduced abdominal compliance (36 versus 62 mL x cmH2O(-1)). Twitch oesophageal pressure was preserved (15.0 versus 15.4 cmH2O). The postprandial state increases twitch transdiaphragmatic pressure, and this should be taken into account when using twitch transdiaphragmatic pressure to follow-up patients or to assess the effects of interventions on diaphragm contractility.