The appearance of a new acetaldehyde-induced hemoglobin fraction, HbA1ach, and the effect of alcohol consumption on it and on the ratio of HbA1ach and glycated hemoglobin, HbA1c, were studied in vivo by cation exchange liquid chromatography. The mean +/- SEM of blood HbA1ach level was 171 +/- 13.10(-3)% of total hemoglobin as measured in 34 male teetotallers. Blood HbA1ach levels of 127 social drinkers (182 +/- 6.10(-3)%) were compared with those of 72 heavy drinkers (213 +/- 8.10(-3)%, p less than 0.01), 79 alcoholics (209 +/- 6.10(-3)%, p less than 0.01) and 16 diabetics (419 +/- 28.10(-3)%, p less than 0.001). HbA1ach correlated positively with HbA1c (p less than 0.001) and negatively with HbAo (p less than 0.001). The ratio of HbA1ach/HbA1c was effective in detecting the alcohol-induced increase in the HbA1ach fraction because the ratio reduced the disturbing effect of glucose. The sensitivity of the HbA1ach/HbA1c ratio was 33% in the heavy drinker group as compared to 40% of gamma-glutamyltransferase and 24% of mean corpuscular volume. The HbA1ach fraction and the HbA1ach/HbA1c ratio seem to be valuable in detecting excessive alcohol consumption in its early phase.