Diversity in group membership can present both advantages and disadvantages for group performance. The authors examined how different percentages in workgroup diversity categories (gender, minority, and persons with disabilities) affected measures of group effectiveness (commitment, overall effectiveness, satisfaction, cohesion, trust, equal opportunity climate, and quality) taken from the Military Equal Opportunity Climate Survey Test Version 3.1 (Talking Paper, 1998). The gender and minority, but not persons with disabilities, categories showed increases in perceived work-group effectiveness at the 11–30% diversity level. Results are discussed in terms of possible optimum diversity levels, expanded measures of diversity, and team development of diverse work groups.