Alternative assessment procedures have made consistent inroads into second and foreign language assessment practices over the last decade. The original impetus for alternative assessment methods has been predicated more on the ideological appeal this approach offers than on firm empirical evidence that alternative assessment approaches actually yield value-added outcomes for foreign and second language learners. The present study addresses the issue of differential language learning growth accruing from the use of formative assessment in direct comparison with more conventional summative assessment procedures in a longitudinal design. Eight cohorts of foreign language learners (N = 2215) participated in this eight-year longitudinal study. Four early cohorts in a 320-hour, four-semester EFL program were assessed with mainly conventional end-of-term summative assessments and tests. A sequence of sixteen EAP courses for these learners produced four time-varying grade point averages indexing stability and changes in achievement over the course of the program. Contrasted with these four cohorts were four latter cohorts of learners who engaged in considerably more formative assessment practices. The products of these formative assessments were also converted into manifest variables in the form of four time-varying grade point averages directly comparable to those generated by the four earlier cohorts. In addition to the series of grade point averages indicating achievement, the participants completed three time-varying EAP proficiency measures. Four research questions are addressed in the study: the comparative reliability of summative and formative assessment products; evidence of parallel changes in achievement differentially influencing proficiency growth; an examination of differential rates of growth in the two contrasted cohorts of learners; direct multivariate tests of differential growth in proficiency controlling for pre-instruction covariates. Analyses of growth curves, added growth ratios, and covariate-adjusted gains indicate that formative assessment practices yield substantive skill-specific effects on language proficiency growth.