Childhood overweight is a significant public health issue in Texas. In response to this problem, the 79th Texas legislature passed Senate Bill 42 (SB 42), which promotes healthier school environments in middle schools, in June, 2005. As a result of SB 42, all Texas public middle schools must require students to engage in at least 30 minutes of daily physical activity or a total of 135 minutes of physical activity per week, beginning in the 2006-2007 school year. The purpose of this study was to determine the barriers and facilitators of implementation and the ultimate impact of this mandate on school PE, child physical activity, and child overweight in Texas public middle schools, and specifically among children living in the Lower Rio Grande Valley (LRGV) of South Texas. Questionnaires with key informants (school principals, PE teachers, and school district PE and health education directors) were conducted in a random sample of middle schools to assess the implementation of SB 42 (2006-2007). In addition, a more detailed evaluation of SB 42 was accomplished with a representative sample of middle schools in the LRGV, a low-income Latino area of the state. Measurements in the LRGV schools included observed and measured child physical activity, self-reported physical activity behaviors, and child overweight (BMI).