• This study was conducted to compare DSM-II and DSM-III in the diagnosis of childhood and adolescent psychiatric disorders. Twenty psychiatrist-raters completed standardized diagnostic questionnaires for 24 actual case histories. This report, the first of four, presents the rater agreement with the "expected diagnosis," ie, the diagnosis that we considered most appropriate for each case. The average rater agreement with the expected diagnosis was less than 50%. It was highest in cases of mental retardation, psychosis, hyperactivity, and conduct disorder. In only five cases did the most common diagnosis of the raters differ from the expected diagnosis. Analyses of these cases and those we selected to present specific diagnostic problems to the raters have produced suggestions to improve the reliability of DSM-III.