Argumentation is increasingly recognized as a fundamental intellectual skill, but evidence suggests that few adolescents or adults are skilled arguers. This article reports on an extended (3-year, twice weekly) intervention designed to afford dense practice in dialogic argumentation to middle-school students from traditionally academically disadvantaged backgrounds, with the goal of supporting development of argumentation skills. Students collaborated with peers who shared their positions on a series of social issues, both in small-group argument-building and reflective activities and in electronic dialogues with a succession of opposing-side peers. Annual assessments of individual argumentative skills on new topics showed students gaining in argumentative discourse skills across all 3 years of the intervention. Continued gains during the 3rd year, however, were concentrated among students who began with the least skill, highlighting the potentially equalizing role of such an intervention.