Aim To study the changes and clinical significances of middle molecular substances(MMS) levels in the plasma of patients with pregnancy induced hypertension(PIH).Methods Plasma MMS concentrations in 82 cases of PIH,98 cases of normal pregnant women and 66 cases of healthy nonpregnant women were measured by an improved ultra-violet ray absorption method.Results Plasma MMS concentrations in healthy nonpregnant women was 2.23±0.20 ku/L,in normal pregnant women were 2.25±0.23 ku/L(prepartum) and 2.24±0.24 ku/L(48 h postpartum),in all PIH were 3.06±0.29 ku/L(prepartum) and 2.86±0.26 ku/L(48 h postpartum),respectively.Plasma MMS contents in mild,moderate,and severe PIH were 2.72±0.21 ku/L(prepartum) and 2.55±0.19 ku/L(48 h postpartum),2.97±0.25 ku/L(prepartum) and 2.76±0.23 ku/L(48 h postpartum),3.20±0.34 ku/L(prepartum)and 3.12±0.30 ku/L(48 h postpartum),respectively.The results showed that plasma MMS concentrations were significantly higher in PIH group than those in normal pregnant women group and healthy nonpregnant women group respectively(p<0.01).There was significant difference among plasma MMS levels from patients with mild,moderate,and severe PIH(p<0.01 or p<0.05).The higher plasma MMS levels,the worser state of PIH.The plasma MMS levels decreased significantly in 48 h post partum in PIH group(p<0.05).There was no obvious difference between plasma MMS levels from normal pregnant women and healthy nonpregnant women(p>0.05).Conclusions The changes of plasma MMS levels are believed to be association with pathogenesis of PIH and can be used as a biochemical marker for judging the progression of PIH.