Waste stabilization has emerged as one of the frequently required best demonstrated available technology (BDAT) treatment methods for land disposal of hazardous wastes as mandated under the Hazardous and Solid Waste Amendments (HSWA) of 1984. Despite this increasing reliance on stabilization as a final treatment for hazardous wastes, there is little published information on the long-term leaching performance for this type of treatment. Recognizing this, Monsanto began a study in early 1988 to evaluate the effect of elapsed time after treatment for a selected group of four Monsanto wastes, which included metallic and reactive solids. The wastes were sent to six stabilization vendors, who returned treated waste samples to Monsanto for analysis. A Toxicity Characteristic Leaching Procedure (TCLP) extraction was performed on each raw waste sample and on each stabilized waste sample at 28 days (in duplicate) and at 90, 200, 470, and 650 days (single samples) after treatment. The results demonstrated a highly waste-dependent effect of time on the TCLP results. Leachate values for some metallic wastes remained stable over time, and for other metallic wastes, leachate concentrations increased over time. These results suggest that additional scientific and regulatory evaluation of stabilization technologies and treated samples may be required to ensure long-term leaching performance confidence.