In a test-retest reliability study involving 25 psychiatric patients and 5 professional raters we demonstrate that research clinicians from collaborating institutions are able to achieve good reliability for most areas of the SADS and RDC when assessing psychiatrically ill patients under interview conditions that provide even less data than ideally obtained in the practice of clinical research. We expect greater reliability in the actual use of the SADS/RDC on most items and diagnoses since the SADS is intended to be used in conjunction with information obtained from relatives, friends, and treatment staff to confirm and clarify the judgements made by the raters on the patient interviews. Moreover, we are reassured that the diagnosis of schizo-affective disorders and schizophrenia is protected from the item unreliability found with specific delusions and hallucinations. Similarly, the difficulties in determining the episodic and chronic nature of the present episode does not substantially interfere with making an RDC diagnosis of the current condition. A complex diagnostic interview system such as the SADS and RDC requires multiple complementary techniques to determine reliability. We find that establishing explicit procedures for raters to discuss and categorize the reasons for their disagreements on individual items and diagnoses provides valuable data for understanding reliability problems. This has helped us to identify specific areas of the interview and criteria that require further clarification and more intensive rater training to improve ratings made by interviewers.