Skin perfusion was measured using laser Doppler fluximetry (LDF) in 16 preterm babies undergoing a standardised heel prick procedure. Although there was a significant reduction in skin blood flow following the heel prick, this was variable and dependent on basal skin blood flow. This, together with loss of data due to movement artefact, makes this technique unreliable in quantifying the sympathetic response to a noxious stimulus in preterm infants.