In response to numerous calls for more rigorous STEM (science, technology, engineering, and mathematics) education to improve US competitiveness and the job prospects of next-generation workers, especially those from low-income and minority groups, a growing number of schools emphasizing STEM have been established in the US over the past decade. However, existing STEM schools vary substantially in the way they are organized, the students they attract, and the outcomes they advertise, and there have been few empirical studies of their effectiveness. This comparative case study examines the opportunity structures for STEM at eight public high schools, four in Denver, Colorado, and four in Buffalo, New York. All of the schools were "inclusive" (no admission requirements) and served predominantly low-income and majority minority students. All but one school had been designated "low-performing" for failure to meet federal accountability requirements. In each city, two of the study schools had recently been re-organized to be "STEM-focused" in some way, and two were traditional, comprehensive high schools. We found that the STEM-focused schools were launched with much enthusiasm and high expectations. In both cities, STEM-focused schools achieved some modest success initially but were unable to maintain their gains. Overall among the schools in this study, the STEM-focused high schools did little to improve STEM opportunities compared to the comprehensive high schools. We do not mean to suggest that STEM schools are a bad idea, but that claims and expectations for them must be examined in the context of their implementation, and STEM schools for low-income and minority students are unlikely to be successful without more attention to systemic issues in urban education.