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Methyl tert -butyl ether (MtBE) has been used as a gasoline additive to increase the octane rating and reduce air pollution in the United States. Due to its high solubility in water, mobility, and low natural biodegradation potential in the subsurface environment, MtBE has become a significant groundwater contaminant. During the last decade, the public, scientific, and regulatory communities have gained a great deal of knowledge about MtBE behavior in the subsurface. This article presents a review of MtBE historical data and its trends over a four-year period at a number of gasoline impacted sites in Los Angeles, California. This article uses field data obtained in the last four years to compare the highest concentration of MtBE at a well with the current concentration at the same well, and with the current highest concentration at the site (may not be the same well as that with the highest historical MtBE value). Statistics of the MtBE data showed that the MtBE historical highest concentrations have decreased between one and two orders of magnitude over the last four years. This result may be explained by the active cleanup at the sites, natural attenuation, and a better management of MtBE. However, the correlation coefficient revealed that a site with a relatively high historical highest concentration (C HH ) may not have a relatively high current concentration (C C ) today at the same well, suggesting that once the source is removed, the center of the MtBE plume may migrate downgradient of groundwater flow. The correlation coefficient also showed that a site with the relatively high C HH relates to current highest concentrations (C CH ) at the site. [ABSTRACT FROM AUTHOR]