Several apo B polymorphic sites have been studied for their potential use as markers for CHD in the population and for potential gene–diet interactions. Our aim was to determine whether the presence of the -516C/T polymorphism in the APOB gene promoter modifies insulin sensitivity to dietary fat. We studied fifty-nine healthy volunteers (thirty men and twenty-nine women, thirty-six homozygotes for the -516C allele (C/C) (nineteen males and seventeen females) and twenty-three heterozygotes for the -516T allele (C/T) (eleven males and twelve females)). Subjects consumed three diets during the feeding study, 4 weeks each: an SFA-rich diet (38 % fat, 20 % SFA), followed by a carbohydrate (CHO)-rich diet (30 % fat, 55 % CHO) or a MUFA-rich diet (38 % fat, 22 % MUFA) following a randomised cross-over design. For each diet, we investigated peripheral insulin sensitivity with the insulin suppression test. Male carriers of the -516T allele showed a significantly greater decrease in steady-state plasma glucose concentrations when changing from the SFA-rich diet (9·18 (sd 1·35) mmol/l) to the MUFA (6·55 (sd 0·74) mmol/l) or the CHO (6·31 (sd 0·93) mmol/l) diets than did those who were homozygous for the C allele (P = 0·040). Furthermore, C/T subjects presented higher plasma NEFA values after consumption of the SFA diet compared with the MUFA and CHO diets (P = 0·001). This effect was not observed in females (P = 0·908). Our findings show that male carriers of the -516T allele, C/T, have a significant increase in insulin resistance after consumption of all diets, but the difference is more exaggerated after the SFA diet compared with the MUFA- and CHO-rich diets.