Previously, it has been concluded that no detectable, adequately determined, acetaldehyde is to be found in the venous blood of normal Caucasian subjects during acute ethanol intake (Eriksson, 1983). Nevertheless, since then a number of reports on human blood acetaldehyde concentrations have been published. Most, if not all, of these levels can, however, be explained as being the result of artefactual sources. Thus, the early conclusion still holds, according to which the concentration of "free" and/or "loosely bound" acetaldehyde is below detection (< 0.5 microM), during normal conditions, i.e., with no deficiency in, or inhibition of, aldehyde dehydrogenase activity. Even if "free" and/or "loosely bound" acetaldehyde cannot be detected, the question of "more firmly bound" acetaldehyde has still remained open, and a number of papers have been published during the last few years on this aspect. Unfortunately, these investigations also have been seriously hampered by artefactual acetaldehyde formations during different hemolysation, hydrolysation, heating and/or other analytical procedures. The appropriate determination of bound acetaldehyde should in the future be better controlled for artefactual formations, even at very low ethanol concentrations (< 0.1 mM). Even if most of the artefactual formation is ethanol-derived, other sources for formation should also be considered. With a view to the present criteria, there is yet no valid demonstration of released acetaldehyde in human venous blood before, during, or after, normal ethanol intoxication.