A method to estimate the probable high groundwater level in Massachusetts, excluding Cape Cod and the Islands was developed in 1981. The method, commonly called the “Frimpter Method,” uses a groundwater measurement from a test site, groundwater measurements from an index well, and a distribution of high groundwater levels from wells in similar geologic and topographic settings. Historic groundwater-level statistics (maximum and 90th percentile groundwater level and annual groundwater-level range) were calculated for 153 wells in Massachusetts and nearby States to update the method inputs. In addition, as part of a comparison of approaches to determine the best index well for a given site, a multiple linear regression equation was developed to explore the relations between predictor variables and the correlations, with the goal of predicting the most highly correlated index well for each test site. This data release includes the calculated groundwater level statistics, the ancillary data used in the regression model, comparison of the data among well pairs, and predictions of the correlations among all well pairs.