The influence of the sudden change of diameter on the local heat transfer in the entry region of tubes was experimentally investigated. An electric direct current was passed through the wall of a tube made of stainless steel and the temperature of the external surface of the tube was measured by infrared thermography. The ratios of diameter were changed between 3/29 mm and 94/29 mm and the Reynolds number between 1000 and 100000. For cross sectional contraction no significant influence of the diameter ratio on the heat transfer could be seen. But for cross sectional extension the heat transfer increased by a factor of eight in comparison to the classical formula for heat transfer in tubes. The maximum heat transfer is a distance of 2 to 3 diameters from the entrance. After this maximum a correlation is given for the Nusselt function with the diameter ratio as a parameter.