A flexible blue LED mattress has been designed and built, it is capable of providing therapeutic blue light irradiance for treating neonatal jaundice. Its 25 × 10 cm size and 60 g weight allows the mother to use it during kangaroo mother care and illuminate the back and posterior part of the neonate head. A method to estimate the thermal energy transfer from the LED mattress to a neonate has been carried out using thermal mannequins in order to assess thermal safety. One mannequin represents a late pre-term neonate of 2.35 Kg, cast on silicone rubber containing 650 ml of water, the other mannequin represents the anterior part of a mother torso with silicone rubber skin; both mannequins are heated by nicrom wire inside them; the power is controlled in a closed loop by means of a microcontroller based system. The mannequins have been arranged in kangaroo mother care position, held to one another by an elastic rubber band making 'skin to skin' contact. Results: Mean blue light irradiance in the contact surface is 350 uW/cm2, the LED matress draws 0.98 Watt from a 12 Volt battery. Heat transfer from the mattress to the neonatal mannequin has been indirectly measured as 0.395 W. Maximum temperature in the contact surface is 2.75 oC above neonatal mannequin skin temperature.