We evaluated the usefulness of case-finding instruments for identifying patients with major depression or dysthymia in primary care settings using English language literature from Medline, a specialized trials registry and bibliographies of selected papers. Studies were done in primary care settings with unselected patients and compared case-finding instruments with accepted diagnostic criterion standards for major depression were selected. A total of 16 case-finding instruments were assessed in 38 studies. More than 32,000 patients received screening with a case-finding instrument; approximately 12,900 of these received criterion standard assessment. Case-finding instruments ranged in length from 1 to 30 questions. Average administration times ranged from less than 2 min to 6 min. Median sensitivity for major depression was 85% (range 50% to 97%); median specificity was 74% (range 51% to 98%). No significant differences between instruments were found. However for individual instruments, estimates of sensitivity and specificity varied significantly between studies. For the combined diagnoses of major depression or dysthymia, overall sensitivity was 79% (CI, 74% to 83%) and overall specificity 75% (CI, 70% to 81%). Stratified analyses showed no significant effects on overall instrument performance for study methodology, criterion standard choice, or patient characteristics. We found that multiple instruments with reasonable operating characteristics are available to help primary care clinicians identify patients with major depression. Because operating characteristics of these instruments are similar, selection of a particular instrument should depend on issues such as feasibility, administration and scoring times, and the instruments' ability to serve additional purposes, such as monitoring severity or response to therapy.