This study examined the test–retest reliability, consensual, convergent and divergent validities, sensitivity, specificity, positive and negative predictive values, and accuracy of the Portuguese version of the Schedule for Affective Disorders and Schizophrenia for School-Age Children-Present and Lifetime version (K-SADS-PL). Eighty-nine children/adolescents (65 psychiatric outpatients and 24 healthy controls) were interviewed with K-SADS-PL and completed measures of depressive and anxiety symptoms. The child’s parent/caretaker completed the Child Behavior Checklist. Good to excellent values were obtained for test–retest reliability and consensual validity. For the convergent validity, moderate correlations between the K-SADS-PL and the corresponding self-report measures were observed. Divergent validity was acceptable for the K-SADS-PL diagnoses. The lowest values of sensitivity, specificity, and accuracy of the K-SADS-PL were 88, 88, and 91, respectively. The Portuguese version of K-SADS-PL proved to be a valid and reliable assessment instrument for children and adolescents, and was sensitive, specific and accurate when diagnosing mood, anxiety, adjustment, and attention-deficit/hyperactivity disorders.