The El Ni¿o/Southern Oscillation (ENSO) is the most prominent interannual fluctuation in the atmosphere and ocean. The associated motion and mass redistribution cause variations in the (solid) Earth's rotation. This paper studies the correlation of ENSO with the length of day (LOD) variations by means of cross correlation in the time domain and complex coherence in the frequency domain. A Monte Carlo simulation is conducted to obtain estimates for the coherence confidence thresholds that are needed in the study. ENSO is represented by the sea level (air) pressure difference between Tahiti and Darwin, Australia, similar to the Southern Oscillation Index. The cross correlation between this ENSO representation and LOD variation on the interannual time scale, roughly from 1 to 10 years, is found to have a maximum value of 0.68 at the LOD phase lag of 2 months during the studied period of 1972--1986. We conclude the following. (1) Most of the interannual LOD variation is caused by ENSO. This extends the atmosphere-LOD relationship into the interannual time scale. (2) The transfer of ENSO's axial angular momentum to the solid Earth lags behind the (Tahiti--Darwin) pressure variation by about 2 months. This places the angular momentum transfer in its proper timing with respect to the evolution of ENSO, providing gross constraints on the modeling of ENSO. The corresponding coherence spectrum, in addition, shows minimum correlation around biennial periods, suggesting the influence of the stratospheric quasi-biennial oscillation on LOD variations.