We studied DSM-III diagnoses made by the lay Diagnostic Interview Schedule (DIS) method in relation to a standardized DSM-III diagnosis by psychiatrists in the two-stage Baltimore Epidemiologic Catchment Area mental morbidity survey. Generally, prevalence estimates based on the DIS one-month diagnoses were significantly different from those based on the psychiatric diagnoses. Subjects identified as cases by each method were often different subjects. Measured in terms of kappa, the chance-corrected degree of agreement between the DIS and psychiatrists' one-month diagnoses was moderate for DSM-III alcohol-use disorder (abuse and dependence combined), and lower for other mental disorder categories. The unreliability of either the DIS or psychiatric diagnoses is one potential explanation for the observed disagreements. Others include the following: insufficient or inadequate information (on which to base a diagnosis); recency of disorder; incomplete criterion coverage; overinclusive DIS questions; and degree of reliance on subject symptom reports. Further study of the nature and sources of these discrepancies is underway. This work should produce a more complete understanding of obstacles to mental disorder case ascertainment by lay interview and clinical examination methods in the context of a field survey.