It is argued that the assumption of negligible temperature gradients across the thickness in blown film extrusion is not valid in the vicinity of the die exit. Numerical simulation of heat transfer involving convection and radiation shows significant temperature differences between the inside and outside film surfaces, which gradually decrease and eventually become negligible near the frost line. The differences are influenced by the magnitude of local heat transfer coefficients. In blown film lines having internal bubble cooling (IBC), temperature gradients in the thickness direction may be reduced, but still they cannot be considered negligible for typical production rates. © 2005 Wiley Periodicals, Inc.