During a series of in-pile experiments designed to study irradiation effects on high-performance oxide fuel elements for advanced gas-cooled reactors, heat-transfer data were obtained from four specially instrumented fuel rods. An annular geometry was utilized with rods of 1.9- and 2.18-cm diam being contained in channels of 2.44- and 2.67-cm diam, respectively. The effects of wire-wrapped and machined square-thread surface roughness were measured and compared with the results obtained from a smooth rod. The fuel rods contained UO2 pellets of varying enrichment and were clad with type-304 stainless-steel tubing. The test parameters for the data reported are: 1) coolant flow rate from 45 to 150 kg/h of helium at 20 atm which corresponds to Reynolds numbers from 15 000 to 45 000; 2) cladding temperatures to 840°C; and 3) heat fluxes from 30 to 100 W/cm2. The smooth-rod data can be correlated with a standard deviation of ±10% by the expression Roughening the rods increased the heat transfer by approximately a factor of 2 with no significant difference between the wire-wrapped and machined roughnesses. The results are generally found to be in good agreement with the results of previous heat-transfer studies. Some consequences of using heat-transfer promoters in nuclear reactor fuel elements are discussed.