The use of standards-based classroom assessments to test English learners' language proficiency is increasingly prevalent in the United States and many other countries. In a large urban school district in California, for example, a classroom assessment is used to make high-stakes decisions about English learners' progress from one level to the next, and as one of the criteria for reclassifying students as Fluent English Proficient. Yet many researchers have questioned the validity of using classroom assessments for making high-stakes decisions about students (Brindley, 1998; 2001; Rea-Dickins and Gardner, 2000). One way to investigate the validity of the inferences drawn from these assessments is to examine them in relation to other measures of the same ability. In this study, a multivariate analytic approach was used to examine the extent to which the English Language Development (ELD) Classroom Assessment measures the same constructs as the CELDT (California English Language Development Test), the statewide standardized test of English proficiency. Using confirmatory factor analysis of multitrait-multimethod data, this study investigates the construct validity of these measures by focusing on evidence of convergence, discrimination, and method effects longitudinally over three years. The study concludes that the evidence gathered via the ELD Classroom Assessment is consistent with that provided by the CELDT, the standardized measure. (Contains 10 tables, 4 figures and 3 footnotes.)