This study estimated the overall rate of use of psychotropic medications among youths (less than 18 years of age) in the U.S. mental health service system and compared rates of use for youths with a single diagnosis and those with co-occurring diagnoses. Results were based on nationally representative client data from the 1997 Client/Patient Sample Survey. On any given day, about one-third (32.5 percent) of the 559,769 youths in the service system received psychotropic medication. Youths with co-occurring diagnoses were significantly more likely to receive medication than youths with a single diagnosis. Future research on best practices may be most effective if the role of medication in the treatment of single and co-occurring disorders among youths is evaluated.