Objectives: To study the association between the mean telomere length (MTL) of human umbilical endothelial cells (HUVEC) and their sensitivity to apoptosis. Methods: Apoptosis of HUVEC was induced by using free hydroxyl radicals. The rate of apoptosis was determined and mean telomere length of HUVEC that were cultured for 1 or 3 months were measured by Southern Blot. Results: At 0.2 mmol/l FeSO(4)/0.0001 mmol/l H(2)O(2) free radical concentration, the apoptosis rate was 8.0 and 17.5% and MTL was 4.66 +/- 0.05 and 3.40 +/- 0.46 kb for HUVEC cultured for 1 and 3 months, respectively. At 0.2 mmol/l FeSO(4)/0.005 mmol/l H(2)O(2), apoptosis rates were 17.4 and 36.0% and MTL were 3.67 +/- 0.06 and 2.90 +/- 0.20 kb for HUVEC cultured for 1 and for 3 months, respectively. Control HUVEC had apoptosis rates of 0.5 and 1.0% and MTL of 5.43 +/- 0.45 and 4.57 +/- 0.21 kb for 1 and 3 months, respectively. The MTL and the apoptosis rates in the treatment groups differed significantly from the controls (p <0.05). Conclusions: HUVEC with less culture time or short telomere were sensitive to oxidation stress. Oxidation stress also can enhance the shortening of telomere length.