Erythrocyte mean cell volume (MCV) correlates well to alcohol intake in moderate alcoholism, but only about 50% of heavy drinkers have increased MCV. To evaluate the influence of the duration and extent of a drinking episode on MCV, 64 addictive alcoholics were investigated prospectively within two weeks after a drinking period. Their daily alcohol intake was 120–480 g and the actual drinking period has lasted for 1–104 weeks. For comparison, 21 non‐active alcoholics were investigated. There was no correlation between MCV of active alcoholics and daily alcohol consumption or smoking habits, whereas a significant positive correlation was found between MCV and both duration of actual drinking episode and total alcohol intake in this period. We conclude that MCV is probably of greater value in estimating the duration and extent of actual drinking episodes in heavy alcoholics than in screening for alcoholism. 1986 Association for the Publication of the Journal of Internal Medicine