Eight previous investigations have suggested that conjoint consideration of findings on tests for gamma glutamyl transpeptidase (GGT) and carbohydrate deficient transferrin (CDT) substantially enhances sensitivity of screening for alcohol problems while minimally diminishing specificity. Using results from a large clinical trial, the current study evaluated the two tests singly and in combination as measures of three clinically important treatment outcome criteria: any drinking, at least one day of heavy drinking, and at least three consecutive days of heavy drinking during the past month. When scored by quartile, CDT is slightly better at screening for alcohol problems in males than GGT. However, CDT seems less accurate in females than GGT. Use of the two tests in consort moderately improves the individual test accuracy in predicting drinking status for both genders.