To estimate the magnitude of stigmatizing attitudes toward peers with depression or attention-deficit/hyperactivity disorder (ADHD) in a national sample of children ages 8 to 18 and to examine variation in level of stigma by school location, region of the United States, grade level, race/ethnicity, or sex. Surveys were administered to 1,318 children and adolescents. Respondents were randomly assigned to depression, ADHD, or asthma conditions and were presented with a vignette about a peer with one of the conditions. Participants responded to items assessing positive and negative attributions, social distance, and family attitudes. Mean differences were tested for statistical significance and effect sizes were computed. Respondents were more likely to make negative attributions about peers with ADHD and depression versus asthma, particularly regarding the likelihood of antisocial behavior and violence (Cohen d range 0.78-1.35, large effect sizes). Moderate effect sizes were found for preferences for social distance from peers with ADHD(d = 0.37) and depression (d = 0.45). Effects were found for perceptions of negative family attitudes toward both mental health conditions, with depression (d = 0.78) seen as even more stigmatized than ADHD (d = 0.47). The level of stigmatization was relatively constant across demographic variables, with the exception of greater stigmatization evident in Asian/Pacific Islander youths. Across most items, both depression and ADHD were more stigmatized than asthma, with depression more stigmatized than ADHD. The perception of likelihood of violence and antisocial behavior was particularly high for both ADHD and depression, greatly exceeding the real-world association for depression.