The current paper describes the prevalence of psychiatric diagnoses in a large sample (n = 150) of adolescents with developmental disabilities who were hospitalized for inpatient psychiatric treatment. Differential diagnoses made during their inpatient stay in a specialty unit for the assessment and treatment of dually diagnosed adolescents are presented and contrasted with longitudinal/historical data on these same patients' diagnoses prior to admission. Results indicate that these individuals received a wide spectrum of diagnoses during their adolescent years. The paper offers indirect support that correctly diagnosing psychiatric conditions is often challenging in adolescents with developmental disabilities. Factors related to diagnostic complexity and misdiagnoses (false positives and false negatives) are discussed. Longitudinal data on psychotropic medication usage for these individuals are also presented.