Groundwater occurrence of ethylene thiourea (ETU) was investigated in conjunction with the National Pesticide Survey (NPS) conducted by the US Environmental Protection Agency (EPA) in 1989 using EPA Method 509. Based on occurrence data collected for this contaminant, EPA estimated that ETU would likely be found in as many as 8,470 (0.1 %) of domestic wells. Studies conducted since the NPS have shown an increased incidence of tumors in rats and mice exposed to ETU leading to its classification as a Group B2, probable human carcinogen. These findings gave rise to new interest in collecting additional occurrence data for ETU using a method with higher sensitivity. The EPA is considering including ETU in a survey that will be conducted under the Unregulated Contaminant Monitoring Rule in 2006. A revision of the method allowed EPA the opportunity to incorporate changes that would improve safety, reduce cost, and make the method easier to perform in the laboratory. The use of a large volume injection (LVI) will reduce the size of the water sample and the amount of extraction reagents used. Two drawbacks of LVI are that injection port conditions must be carefully optimized and that impurities from glassware and extraction reagents, which may not be significant at low-volume injection, become problematic. Includes 8 references, tables.