An EOF-based reconstruction of monthly SST anomaly fields is calculated for the years 1868–1993 in the tropical Pacific Ocean. The EOFs are computed from optimally interpolated SST anomaly fields from November 1981 to October 1995 from the same region. These are used as a functional basis set for projecting SST anomalies from the Comprehensive Ocean–Atmosphere Data Set (COADS) to produce a smooth, spatially complete reconstruction. The optimal number of EOF modes for each month’s reconstruction is chosen using large-scale error analysis: a modern subset of the COADS SST is subsampled according to the distribution of observations in the historical month being reconstructed, forming a testbed on which to evaluate the reconstruction based on a variable number of modes. The test bed is least squares fit to the basis EOFs. The difference fields between the reconstruction and a smoothed version of the test bed is found. The total variance of difference fields (residual) is minimized with respect to the number of EOF modes. Additionally, the residual is not allowed to be greater than the total variance of the large-scale features of the test bed. The number of modes used in each monthly reconstruction increases with the spatial coverage of observations. The time series associated with the first EOF mode is a new ENSO index. There is general agreement of warm and cold ENSO events between this new index and those determined by the SOI as well as those described in previous studies. A wavelet analysis of the new index reveals its spectral evolution from 1870 to 1992 is similar to that of the SOI. Most of the energy at periods less than 10 yr is concentrated in the 3–8-yr range in bursts typically lasting 5–10 yr. Previous reconstruction studies also show agreement with this new ENSO index as measured by the timing and amplitude of premodern El Niño events.