The temporal variability of Pacific Ocean upper ocean heat content is examined for the 1948-1998 period using gridded, objectively analyzed temperature anomaly fields. Results indicate a "regime shift" in upper ocean temperature structure occurred in conjunction with the atmospheric shift in sea level pressure around 1975 leading to warming (cooling) in the eastern (western) equatorial Pacific. The shift is a basin-wide phenomenon effecting the thermal structure from 60°S to 70°N. EOF analysis of the Pacific Ocean heat content (0-125 m) shows a shift from a relatively cool to a warm state in the equatorial Pacific Ocean during the mid-1970s. Further analysis of the gridded temperature anomaly fields shows equatorial warming to be as much as 1.5°C and a cooling in the North Pacific of 1°C, down to 250 meters depth after the mid-1970s. Overall, the analysis indicates the "regime shift" continues through 1998 with no signs of returning to a cooler phase.