We have recently shown that an inherited polymorphism occurring in the human apolipoprotein A-I (apo A-I) gene is related to decreased high density lipoprotein and apo A-I levels in the plasma of two patients with severe premature atherosclerosis. Analysis of the molecular basis of this polymorphism and its possible effects on apo A-I gene expression requires direct comparison of both normal and polymorphic apo A-I alleles. Here we report the isolation and characterization of the normal human apo A-I gene and we show that the gene is interrupted by three intervening sequences, IVS-1, IVS-2, and IVS-3, occurring in the 5' noncoding region of apo A-I mRNA, the mRNA sequence coding for the signal peptide of apo A-I, and the sequence coding for the mature protein, respectively. In addition, the nucleotide sequence analysis of the apo A-I gene allowed determination of the complete amino acid sequence of the primary translation product of apo A-I mRNA. This amino acid sequence consists of 267 residues including a 24-residue-long amino-terminal extension (preprosegment). Finally, we show that the apo A-I gene contains six 66-base-pair-long tandemly repeated DNA segments, which suggests that the gene may have evolved by intragenic duplication events.