Abstract： Objective To discuss that the outbreak of recurrent aphtha has relation to the rise of serum NO, NOS level. Methods From 1999 to 2000,we compared the serum NO and NOS levels of 120 recurrent aphthae patients to 50 healthy people, and analyzed the differences of the serum NO and NOS between the two groups.Results For the serum NO level and NOS level:In the outbreak phase,the figures are 115 ± 19.6μmol/L、42.27±5.58μmol/L respectively; in the diapauses of recurrent aphtha,they are 81.26 ± 12.15 ,36.12 ± 7.40 seperately; in the healthy contrast group, they are 70.4 ± 16.3μmol/L and 32.8 ± 7.8μmol/L. The indexes of the outbreak phase group are higher than those of the healthy contrast group, tNO = 2.736 ,PNO ＜ 0.005. tNOS =4.205 ,PNOS＜ 0.005, and they have significant differences. The indexes of the outbreak phase are also higher than those of the diapauses,tNO = 16.027 ,PNO ＜0.005. tNOS =7.269,pNOS ＜0.005 ,notable differences exist. Conclusion Thus,the rise of serum NO, NOS level is involved in the occurrence of recurrent aphthae.