Endothelial cells (ECs) undergo a finite number of cell divisions before growth arrest or replicative senescence; this is modulated in part by the proinflammatory cytokine, interleukin-1 (IL-1). IL-1 and its family members are expressed in human atherosclerotic vessels, mainly in the endothelium. EC replicative senescence and IL-1 have been associated with atherosclerosis. Genetic variants at the IL-1 locus have been associated with a variety of coronary phenotypes. In this study, we examined the relationship between the interleukin-1 receptor antagonist variable number tandem repeat allele 2 (IL-1RN*2*2) and EC replicative capacity. A significant decrease in EC cumulative population doublings (CPDs) was associated with the rare allele (IL-1RN*2*2) at IL-1RN, 8.56±0.97 (n=7) versus 13.14±1.00 (IL-1RN*1*1, n=20), P=0.0118. Proliferation of IL-1RN*2*2 ECs detected by Ki67 expression was also significantly reduced particularly at later passage, passage 6: 21.76±0.93% (n=6) versus 48.10±8.81% (IL-1RN*1*1, n=7) (P=0.0323) and passage 8: 22.48±3.08% (n=6) versus 42.29±3.06% (IL-1RN*1*1, n=7) (P=0.0028). IL-1RN*2 carriage was associated with increased numbers of senescent ECs detected by senescence-associated -galactosidase staining. Basal apoptosis, telomerase activity, and telomere length were not different with respect to IL-1RN genotype. Addition of exogenous IL-1ra (1 ng/mL) increased CPDs in a number of human umbilical vein endothelial cell cultures and increased proliferating cells from 12.11±1.21% to 27.82±2.82% (P=0.0216) with IL-1RN*2*2 at passage 8 (n=2). These data suggest genetic control of EC proliferation and life span by the IL-1 locus and imply that IL-1ra may have a function connected with EC growth.