Purpose: To evaluate the prevalence of ICD-10 psychiatric disorders in a population of pregnant teenage women from a Brazilian public hospital. Method: 1000 pregnant teenage women were evaluated using the Composite International Diagnostic Interview, a structured interview which establishes diagnoses according to the International Classification of Diseases. Results: 325 of the 1000 patients were found to have at least one psychiatric disorder; 33.2% of them had a second psychiatric diagnosis. The most prevalent disorders were depression (12.9%), post-traumatic stress disorder (10%), and tobacco dependence/harmful use (10.3%). The most prevalent co-morbidity was ICD-10 Affective Disorders versus Neurotic, Stress-related, and Somatoform Disorders. Substance-related disorders are significantly correlated with all of the ICD-10 psychiatric diagnoses but Psychotic Disorders. Conclusion: The prevalence of psychiatric disorders in this population is high, and one third of them had co-morbidities, a condition usually associated with a more serious course of illness. This finding has important implications for clinical management and prognosis, and demands preventive public policies. (Contains 2 tables.)