This study examined changes in the prevalence of psychiatric diagnoses at admission among children and adolescents treated for mental health problems in psychiatric inpatient settings between 1995 and 2000. Using a large, nationwide database (MarketScan) of private health insurance claims, our sample consisted of 5,346 children under the age of 18 who received psychiatric inpatient services, out of a total of 1,723,681 covered children. Odds ratios were used to measure changes in the prevalence of specific mental health disorders between 1995 and 2000. The study identified several significant changes, most notably, that the proportion of hospitalized children treated for bipolar or eating disorder doubled between 1995 and 2000. Significant decreases were observed for adjustment, anxiety, oppositional, and substance abuse disorders. This study lends support to recent concerns that the prevalence of bipolar disorder among the youth is increasing. Further research is needed to identify the underlying reasons for these observed changes.