Only a few studies have analyzed the effects of the potential interaction between the -174G/C polymorphism of IL6 gene and the adherence to the Mediterranean diet (MD) on adiposity indexes. Our aim was to investigate the interplay between the -174G/C polymorphism of the IL6 gene and a Mediterranean-style diet on body weight changes after 3 years of nutritional intervention in a high cardiovascular risk population. A total of 737 participants, aged 55-80 years were assigned to a low-fat diet or to a Mediterranean-style diet group with high intake of virgin olive oil (VOO) or nuts. Anthropometric measurements were taken at baseline and after 3-year follow-up. The -174G/C polymorphism of the IL6 gene was genotyped. Minor allele frequency (C) was 0.39. At baseline, the CC genotype was associated with higher measures of adiposity. After 3 years, a significant interaction (p = 0.028) was found between the polymorphism (GG+GC versus CC) and the nutritional intervention: CC subjects following the MD+VOO had the lowest body weight gain. In conclusion, at baseline, CC subjects for the -174G/C polymorphism of IL6 had the highest body weight and BMI. However, after 3 years of nutritional intervention with MD+VOO, these subjects were predicted to have the greatest reduction in body weight.