Background: investigation of the relationship between lactate level in vaginal fluid and the latent phase of labour in pregnancies is complicated by preterm premature rupture of membranes (PPROM). To assess whether lactate determination in vaginal fluid can predict, onset of labour for women with suspected prelabour upture of the membranes (PROM). Patient and Methods: 110 pregnant women with history of PPROM recruited from the outpatient clinic and emergency room in EL Hussein Hospitals and Om El Masreen Hospital during the period from February 2018 till August 2018. All women underwent sterile speculum examination; then 5 ml of sterile saline solution was injected in the posterior vaginal fornix using a sterile speculum and 2ml of the injected saline was withdrawn with the same syringe for lactate Measurement. Results: the best cutoff point of vaginal fluid lactate concentration as a predictor of onset of labor within 48 hours after PPROM was 4.7 mmol/L above which the onset of labor within 48 hours was likely with a sensitivity of 95.4%, specificity of 91.3%, a positive predictive value of 97.7%, a negative predictive value of 84%, an overall accuracy of94.6%, a positive LR of 10.97 and a negative LR of 0.05. Conclusion: lactate determination in vaginal fluid seems promising as a tool to predict onset of labor within 48hours in women with PPROM. The best cutoff point of vaginal fluid lactate concentration as a predictor of onset of labor within 48 hours after PPROM was 4.7 mmol/L.