Although the average differences between invasive and non-invasive BP measurements are acceptable, the range of under- and overestimation of non-invasive BP measurements is large and not consistent, making reliance on non-invasive modalities to guide circulatory management problematic. If arterial BP monitoring is not available, our results suggest measuring non-invasive BP on the leg in preterm infants with a birth weight no greater than 1,000 g.