Despite limited empirical evidence, there is growing concern that junk food availability in schools has contributed to the childhood obesity epidemic. In this paper, we estimate the effects of junk food availability on body mass index (BMI), obesity, and related outcomes among a national sample of fifth graders. Unlike previous studies, we address the endogeneity of the school food environment by controlling for children's BMI at school entry and estimating instrumental variables regressions that leverage variation in the school's grade span. Our main finding is that junk food availability does not significantly increase BMI or obesity among this fifth-grade cohort despite the increased likelihood of in-school junk food purchases. The results are robust to alternate measures of junk food availability including school administrator reports of sales during school hours, school administrator reports of competitive food outlets, and children's reports of junk food availability. Moreover, the absence of any effects on overall food consumption and physical activity further support the null findings for BMI and obesity. (Contains 33 footnotes and 16 tables.)