The effect of UO2 density on fission product gas release and sheath expansion has been determined in an irradiation experiment in which the performance of fuel elements with densities between 10.42 and 10.74 g/cm3 was compared at ∫λdθ values of 39 and 42 W/cm. Diametral sheath strain was less for the lower density elements, but fission product gas release and the extent of grain growth were greater as the density decreased. A correlation between the extent of grain growth in the UO2 and the fractional gas release was found to exist in this test and to apply to a large number of other irradiations; it is suggested that this relationship may be causal and that the fractional gas release is not solely a function of temperature.