High levels of low density lipoprotein (LDL) and its apolipoprotein B (apoB) are risk factors for atherosclerosis and myocardial infarction (MI). There is rich genetic polymorphism in apoB, first detected as the Ag allotypes of LDL, but today mostly examined at the DNA level. Genes contribute to the population variation in LDL and apoB levels and alleles in polymorphisms at the apoB locus are candidate genes with respect to control of lipid levels and susceptibility to atherosclerosis and MI. The XbaI polymorphism at the apoB locus, which involves the third base of threonin codon 2488 (ACC-->ACT) without changing the amino acid sequence was examined in a case-control study comprising 238 survivors of myocardial infarction (MI) and 621 controls. In univariate analysis, frequencies of genotypes in this polymorphism were not statistically different between patients and controls of either sex. However, in multivariate logistic regression analysis, the odds ratio X-X- homozygotes (homozygotes for absence of restriction site) for having MI compared to the pooled group of heterozygotes and X+X+homozygotes (homozygotes for presence of restriction site) was 2.16 (p = 0.007), after adjustments for age, sex, and levels of apoB, high density lipoprotein (HDL) cholesterol (HDLC) and Lp(a) lipoprotein. It appeared that heterozygotes do not have increased risk, compared to the X+X+ homozygotes. Stratification according to low or high levels of apoB, HDLC and Lp(a) lipoprotein, showed that the X-X- genotype was more common in patients than controls, in all subgroups.(ABSTRACT TRUNCATED AT 250 WORDS)