'Damping' of an intra-arterial pressure waveform theoretically affects systolic and diastolic blood pressure whilst mean blood pressure (mbp) continues to be reliable. We assessed changes in mbp associated with 79 episodes of damping in 30 infants. The average mbp in the hour just before damping occurred was compared with that in the first hour of damping. The mean change in mbp was -0.845 mmHg. In 50% of cases this change was < 2 mmHg and in 24% of episodes > or = 4.1 mmHg. Damping may produce clinically significant changes in mbp which computer trend data may help to differentiate.