This paper presents the results of experimental investigation of convection heat transfer from horizontal rotating cylinders to ambient air. Two hollow copper cylinders were used, one 4.5 in. and another 3.25 in. outside diameter. These cylinders were nickel plated to minimize the heat transfer by radiation and were internally heated by electric cartridge heaters. The surface temperatures were determined by thermocouples peened to the inside surface. The experimental procedure was to investigate systematically each dimensionless parameter which was previously determined by dimensional analysis. The results indicate that, up to a certain value of Reynolds number, rotation has no effect on the heat-transfer coefficient. Above this critical value the heat-transfer coefficient increases as the speed of rotation increases. In the region where the data overlap, the experimental results reported in this paper are in close agreement with those of other investigators. The flow pattern around the cylinder was also investigated. Titanium tetrachloride smoke was used for this study.