The General Health Questionnaire (GHQ-12) (Goldberg, 1972) and the Self Report Questionnaire (SRQ-20) (Harding et al. 1980) were simultaneously validated against the criterion of the Clinical Interview Schedule (CIS) (Goldberg et al. 1970) in three primary care clinics in the city of Sao Paulo, Brazil. A comparison between the two screening instruments was carried out. The product-moment correlation between the two sets of scores was +0.72, and the validation coefficients for the GHQ-12 and the SRQ-20 were respectively: sensitivity 85% and 83%; specificity 79% and 80%; overall misclassification rate 18% and 19%. The two screening instruments were further compared by the application of Relative Operating Characteristic (ROC) analysis and, again, were found to be very similar in performance. The area under the ROC curve was 0.87 for the GHQ and 0.90 for the SRQ-20 (Z = 0.92, NS). Both psychiatric screening instruments were therefore found to be acceptably valid when assessed against the CIS in three primary care settings in Brazil. The application of ROC analysis to studies of this kind was feasible and straightforward. It was found to be superior to the conventional method of presentation of validity data. It is recommended that the application of ROC analysis to psychiatric screening studies be further utilized and explored.