A new apolipoprotein A1 (APOA1) gene variant has been identified in a family ascertained through a proband undergoing coronary angiography. The variant, ApoA1 Baltimore, was due to a mutation at codon 34 of the third exon of the APOA1 gene (CGA to CTA) that resulted in an arginine-to-leucine substitution at the tenth amino acid of the mature ApoA1 and a change in charge of -1. The mutation abolishes a TaqI restriction site and it is easily detectable after polymerase chain reaction amplification of genomic DNA. The proband was heterozygous for the mutation. Eight other members of the pedigree had the same ApoA1 variant. Cosegregation of the variant with hypoalphalipoproteinemia could not be demonstrated and the association of this mutation with hypoalphalipoproteinemia was confined to three affected members of the nuclear family. No effect of the mutant on any lipoprotein phenotype could be established.