Fresh blood samples were collected from 103 North Karelians who had in 1981-84 participated in dietary intervention studies and analysis of the XbaI restriction fragment length polymorphism (RFLP) of apolipoprotein B (apoB) was carried out. Reanalysis of the original plasma lipid and apolipoprotein data indicated that while baseline concentrations did not differ significantly between genotypes, the response to a low-fat, low-cholesterol diet was influenced by apoB XbaI genotype: reductions in total and low density lipoprotein (LDL) cholesterol, apoB and high density lipoprotein (HDL) cholesterol were greater in subjects homo- or heterozygous for the presence of the XbaI cutting site (X1X2 or X2X2 genotype, designated X2+) as compared to those lacking the cutting site (X1X1 genotype, designated X2-). The corresponding average reductions induced by dietary intervention in X2+ and X2- subjects were: for total cholesterol 1.30 and 0.99 mmol/l (p = 0.036), for LDL cholesterol 1.04 and 0.78 mmol/l (p = 0.049), for apoB 18.3 and 8.1 mg/100 ml (p = 0.012) and for HDL cholesterol 0.26 and 0.17 mmol/l (p = 0.008).