Flectalon, web of aluminised polyvinylchloride fibres, has been formulated to minimise radiant heat losses and to provide conventional insulation. Critical temperature determinations were used to assess the insulating efficacy of this and other swaddling materials in infants. The critical temperature for a baby 2 to 10 days old was 31 degrees C when naked and 23 degrees C when wrapped in a Silver Swaddler or a sheet and two blankets. The use of a quilt made with Thinsulate or Hollofil with a mass per unit area of 160 to 180 g/m2 reduced the critical temperature to 19.5 degrees C, while Flectalon of comparable weight reduced the critical temperature to 13.8 degrees C: Flectalon is thus an efficient insulator. The risk of overheating was studied by monitoring swaddled babies, rectal temperatures at various ambient temperatures. Some forms of swaddling caused increases in rectal temperatures at "normal' hospital temperatures, implying risks from warmer environments and assessments of swaddling materials should, therefore, include medical evaluation of efficiency and safety. Flectalon merits assessment in other groups at risk from hypothermia.