In a sample of 3,498 students living in two counties in California, each student's weight and backpack load were measured. Demographic information as well as information about backpack use was obtained. Nonspecific mechanical back pain was found to be highly prevalent, and the reported severity and chronicity of pain were high. Controlling for age, socioeconomic status, walking to and from school, and method of wear, results indicated that backpack weight, measured as a percentage of body weight, was effective in predicting back pain (P < 0.01). Girls and those who walk to and from school were more likely to report back pain (P < 0.01). The method of wear, socioeconomic status, and age were not found to be significantly related to the prevalence of back pain. However, with regard to the severity of pain, older age (P < 0.01), walking to and from school (P < 0.01), and method of wear (P < 0.05) were statistically significant.