To compare the endotoxin levels between acute and chronic periapical periodontitis with different clinical symptoms.10 cases of acute periapical priodontitis(Group 1), 10 cases of chronic periapical periodontitis (group 2, the diameter of apical radiolucency area was less than 2 mm), 10 cases of chronic periapical periodontitis with sinus(group 3, the diameter of apical radiolucency area was greater than 2 mm), 10 cases of chronic periapical periodontitis without sinus (group 4, the diameter of apical radiolucency area was greater than 2 mm), were included in the study. Chromogenic substrate method of limulus amebocyte lysate(LAL) test was used to measure the endotoxin level.Endotoxin concentrations in group 2 were significantly lower than those in group 1, group 3 and group 4(P < 0.01).Endotoxin plays a very important role in the initiation and development of periapical periodotitis and is closely associated with clinical symptoms and apical radiolucency degree.