The impact of the El Niño-Southern Oscillation (ENSO) on temperature extremes is examined in both observations and coupled climate model simulations. A recently developed observed gridded dataset of climate extremes indices (HadEX2)shows marked contrasts in seasonal composites of the monthly maximum value of daily maximum temperatures during the cold and warm phases of ENSO. Extreme maximum temperatures are significantly cooler over Australia, southern Asia, Canada and South Africa during strong La Niña events compared to El Niño events and significantly warmer over the contiguous United States and southern South America. Two versions of the DOE-NSF Community Climate System Model (CCSM3 and CCSM4) are contrasted for their ability to capture these relationships given their very different simulations of ENSO; the CCSM3 ENSO has a strong biennial frequency that is too narrowly confined along the equator, while the CCSM4 ENSO is more realistic in both frequency and pattern. While both models capture some aspects of the observed regional changes across the globe, the fidelity of the ENSO simulation appears to be crucial for simulating the magnitude and sign of the extreme maximum temperature relationships. Over the US in particular, the CCSM3 composite pattern of maximum temperature extremes with ENSO is weak and opposite in sign to that observed. The CCSM4 is much improved over the CCSM3, capturing the observed increase in US maximum temperature extremes during La Niña with realistic amplitude, pattern and statistical significance. In a future emissions scenario of the CCSM4, the contrast between maximum temperature extremes during El Niño and La Niña events strengthens over Australia whereas it weakens slightly over the US. Further understanding of the mechanisms leading to these projected changes will be the subject of further work.