The article reports findings in a study of 198 subjects attending a psychiatric out-patient clinic, with known or suspected high alcohol consumption during a 12-month period, about half of whom had had their driving licences revoked. The level of carbohydrate-deficient transferrin (CDT) was found to be a valuable marker of alcohol consumption, and a useful adjunct to the measurement of liver enzymes. Both GGT (gamma-glutamyltransferase) and CDT levels were significantly higher in high alcohol consumers than in low consumers. Alcohol the two markers did not differ from each other in statistical significance, CDT was associated with greater sensitivity and specificity; the sensitivity of CDT was 69% for men and 79% for women, as compared with 62% and 40%, respectively, for GGT; the specificity of CDT was 81% for men and 100% for women, as compared with 82% and 72%, respectively, for GGT. Together, GGT and CDT detected 91% of the male and 93% of the female high consumers. Among younger men, CDT values were higher in the subgroup with a history of traffic offences than in the subgroup without such a history, thus suggesting that CDT levels may be increased by heavy weekend beer consumption. A few cases of false-positive CDT results were found to be attributable to genetic anomalies of the transferrin molecule. Cases characterised by disparity between the CDT level and the clinical picture require further, more specific, analysis. Used in combination with GGT, CDT is thus a feasible marker for use in monitoring alcohol consumption in drivers needing to qualify for the restoration of their licences.