Hemodynamically unstable patients will benefit from therapy to control their blood pressure only if this pressure can be determined with accuracy. We compared the accuracy of arterial blood pressure measurements taken aboard helicopters by palpation with a Doppler device and with an oscillometric device. Twenty critically ill patients who required an intra-arterial line had simultaneous blood pressures taken invasively and with one or more of the nonivasive techniques. A total of 222 comparisons was made. Error measurements are reported as mean +/- standard deviation; a high standard deviation implies inaccuracy in individual noninvasive measurements. Mean palpation error was 19 +/- 22 torr, mean Doppler error was 8 +/- 17 torr, mean oscillometric systolic error was 0 +/- 33 torr, and mean oscillometric mean blood pressure measurement error was 10 +/- 15 torr. A blood pressure undetectable by noninvasive means was no guarantee of a low arterial pressure: four patients with arterial systolic pressures from 45 to 138 torr had nonpalpable pulses; three patients had systolic pressures between 98 and 143 torr that were not detectable with the Doppler device; and two patients had systolic pressures of 70 and 110 torr that were undetectable by the oscillometric device. We conclude that aboard helicopters some ill patients cannot have accurate noninvasive measurement of their arterial blood pressure made with present technology. Relying on inaccurate blood pressure measurements may lead to therapeutic mishaps.