Currently, a substantial part of clinical practice is involved with health maintenance and disease prevention. However, there continue to be deficiencies in the delivery of preventive care, and it is sometimes difficult to identify which interventions are effective--for lack of randomized clinical trials--and the theoretical basis of this approach is not always clear. In the previous paper published in the Journal, we presented the general conceptual principles of preventive medicine, starting with levels of prevention and followed by the criteria for selecting a preventive intervention. Regarding the latter, the characteristics of the disease to be screened and the respective screening test were discussed, as well as therapy for positive cases. Finally, we discussed the possible consequences of screening for disease. In this paper we present guidelines for the critical appraisal of articles on screening.