Mean arterial pressure (MAP) is the area under the pressure wave form averaged over the cardiac cycle. A widely used rule of thumb to estimate MAP of peripheral arterial pressure waves in adults is adding one-third of the pulse pressure (PP) to diastolic arterial pressure (DAP). However, radial artery pressure waves in newborns differ from those in adults and resemble proximal aortic pressure waves, so that the above-mentioned calculation of MAP may not be correct. The present study was set up to obtain an arithmetical approximation to derive MAP from blood pressure waves measured in the radial artery of the neonate. We accurately recorded about 300 invasively obtained blood pressure curves in the radial artery of 10 neonates admitted for intensive care. We found that MAP in the radial artery in these neonates can be well approximated by adding 46.6% PP to DAP (range 43.0-50.1%). We suggest that the rule of thumb to derive MAP from radial artery waves in the neonate to be approximately the average of systolic and diastolic pressure, as opposed to adding one-third of the pulse pressure to the diastolic value in the adult.