This study examined the extent to which clinical and demographic characteristics predicted outcome for children with autism spectrum disorder. Participants included 152 students with autism spectrum disorder in 53 kindergarten-through-second-grade autism support classrooms in a large urban public school district. Associations between child characteristics (including age, language ability, autism severity, social skills, adaptive behavior, co-occurring psychological symptoms, and restrictive and repetitive behavior) and outcome, as measured by changes in cognitive ability following one academic year of an intervention standardized across the sample were evaluated using linear regression with random effects for classroom. While several scales and subscales had statistically significant bivariate associations with outcome, in adjusted analysis, only age and the presence of symptoms associated with social anxiety, such as social avoidance and social fearfulness, as measured through the Child Symptom Inventory-4, were associated with differences in outcome. The findings regarding the role of social anxiety are new and have important implications for treatment. Disentangling the construct of social anxiety to differentiate between social fearfulness and social motivation has important implications for shifting the focus of early treatment for children with autism spectrum disorder.