This study examines elementary school nurses' perceived efficacy expectations, perceived barriers, and perceived benefits to measuring body mass index (BMI) in students in schools with mandated BMI policies versus schools without mandated policies. Of the 2,629 school nurses participating in the study, 67% believe nurses should measure BMI in schools, 62% believe BMI results should be sent home, and 81% believe nurses should be designing programs to help children address overweight issues. In schools with a BMI assessment mandate, the nurses' efficacy expectations are statistically significantly greater and nurses' perceptions of barriers to measuring BMI are significantly lower. The most common perceived barriers to measuring BMI are inadequate school resources (57.6%) and inadequate or inappropriate parental responses (55.2%). Obese school nurses perceive significantly more barriers to measuring youths' BMIs than do nonobese school nurses. Having mandates has a positive influence on school nurses and their measurement of BMI and increases the likelihood that nurses will measure BMI. (Contains 7 tables.)