Most of the literature data on the oxidation of Zircaloy-4 in steam at high temperatures was generated at atmospheric pressure, but what little data does exist at elevated pressures indicates that oxidation may be enhanced. In view of the paucity of available data, a test program was undertaken to determine the oxidation characteristics of Zircaloy-4 in steam at high temperatures and pressures. Lengths of Zircaloy-4 were heated to temperatures between 750 and 1000°C while being exposed to flowing steam at pressures up to 18.6 MPa for durations up to 2500 s. Oxide thicknesses were measured by metallography that indicated that oxidation was enhanced by pressure over this temperature range. The deleterious effect increased with temperature up to 900°C where the oxide thicknesses were up to five times greater than expected from atmospheric pressure data. Where breakaway oxidation occurred, the local oxide thicknesses were up to a factor of seven greater than predicted. At 1000°C, the effect was less pronounced than at 900°C, with maximum enhancement factors of around two. At 800°C, the enhancement saturated around 15.2 MPa, when further increases in pressure to above normal pressurized water reactor (PWR) operating pressure had no further noticeable effect. Scatter in the data at 900°C prevented identification of any firm trends but the indications were that here the effect did not saturate in the pressure range considered. Thus, predictive models based on atmospheric pressure data are likely to substantially underestimate the extent of oxidation when Zircaloy-4 is heated in elevated pressure steam. These results, which will enable more confident estimates of oxidation under such conditions and form a good starting point for empirical modeling efforts, are reviewed in the context of existing literature data and computer models.