AG-->A transition at -78 base pairs from the transcription start site of the apolipoprotein A-I (apoA-I) gene has been associated with increased apoA-I serum levels in humans. We report here that this mutation (G-->A) increases significantly (5-7-fold) the expression of a reporter gene fused to the apoA-I promoter in human liver and intestine cells. In addition, the presence of A at -78 base pairs from the transcription start site of the gene significantly decreases the binding affinity of a nuclear factor present in liver and intestine cells. We suggest that the reduced affinity of this factor increases the transcription efficiency of the promoter and explains why individuals carrying the A allele have high serum apoA-I levels.