This study was designed to assess whether the XbaI restriction fragment length polymorphism (RFLP) for apolipoprotein B (apo B) gene could be related with a genetic predisposition to develop hyperlipidemia and atherosclerosis. Relationships between XbaI RFLP and serum cholesterol were evaluated by comparing hyperlipidemic patients with healthy controls. Statistical analysis (chi-square test) showed no significant difference in either genotype distribution or allele frequencies. Hyperlipidemic patients were then divided according to triglycerides, either above or below 200 mg/dl and XbaI genotype frequencies were measured. No significant differences in genotype distribution or allele frequencies were found. The hyperlipidemic patients were tested for the presence of arterial disease by echo-Doppler and angina questionnaire. The XbaI genotype frequencies were determined in patients with arterial disease and compared to those without evidence of disease. No significant differences were found between the two groups.