Developmental screens are often used in nonstandardized conditions, such as pediatric waiting rooms, despite validation under standardized conditions. We examined the reproducibility of the Ages and Stages Questionnaire (ASQ), a developmental screening instrument commonly used in pediatric practices, under standardized versus nonstandardized conditions in an underserved population. English- or Spanish-speaking parents of 18- or 30-month-old children completed the ASQ in the waiting room and then were randomized to repeat the ASQ in waiting room (W-W) or standardized (W-S) conditions. We calculated ASQ fail rates and intraclass correlation coefficient, a measure of reliability, for each of the 5 domains of the ASQ. We hypothesized that intraclass correlation coefficients in the W-W condition would demonstrate greater reliability than in the W-S condition. A total of 131 parents were randomized (66 W-W, 65 W-S). Parents were mostly minority. Of the entire sample, 25.8% failed the first ASQ screen completed in the waiting room before randomization. There was no statistically significant difference in fail rates between study arms on the first or second screen. Intraclass correlation coefficient for W-W in the 5 domains ranged from 0.66 to 0.95, and for W-S from 0.73 to 0.92. There were no statistically significant differences between intraclass correlation coefficients in W-W versus W-S in any domain. In an underserved population, 25.8% of children failed the ASQ. The ASQ, when completed in the waiting room, is reliable compared with standardized conditions, indicating that the ASQ can be used to screen children for developmental delay in the waiting room of pediatric practices.