Carbohydrate-deficient transferrin, CDT, had previously been reported to be an excellent marker for alcoholism. The present population-based study examined the diagnostic value of CDT among consecutive middle-aged males including 122 social drinkers (mean alcohol consumption 88 +/- 79 g per week) and 77 non-alcoholic heavy drinkers (301 +/- 195 g/wk). Ninety-six men with a well-documented history of chronic alcoholism (> or = 1000 g/wk) were used as a reference group. The CDT (containing mainly isotransferrin with pI = 5.8 and 5.9) was separated by anion exchange chromatography and assayed by RIA. The CDT values of social drinkers (mean +/- SD = 14 +/- 5 U/I) were significantly lower than those of heavy drinkers (19 +/- 13 U/I, p < 0.01) and alcoholics (34 +/- 18 U/I, p < 0.001). In the whole material CDT correlated positively with alcohol consumption (r = 0.53, p < 0.001). At a specificity of 91.8%, CDT found 28.6% of the heavy drinkers and 79.2% of the alcoholics; the best traditional marker, GGT, with a specificity of 86.9%, found 35.1% and 64.6%, respectively. In conclusion, CDT is a specific marker, which is superior to traditional markers for identifying alcoholics. Unfortunately, it does not seem to provide additional power for identifying the important group, non-alcoholic heavy drinkers.