Objective: To determine whether misoprostol-related tachysystole is due to the cumulative effect of the drug. Methods: A group of preeclamptic patients were chosen and data were compared to non-preeclamptic patients to prove the hypothesis. Among 90 patients evaluated, 45 were preeclamptic while 45 were non-preeclamptic. Six out of 43 preeclamptic patients were diagnosed as having HELLP syndrome. All patients were induced by 50 µg of misoprostol applied to the posterior vaginal fornix with 4-hour intervals for a maximum of three times. Results: Two patients from both groups were left out of the study leaving 86 patients to evaluate. The number of misoprostol tablets used in the preeclamptic group was significantly higher (2.32±0.64 preeclamptic vs 1.81±0.76 tablets non-preeclamptic group; [p=0.001]) , while the number of women giving birth after only one tablet of misoprostol was significantly lower (2/24 pre­eclamptic vs. 14/32 non-preeclamptic group [p=0.006, OR 0.116; 95% CI 0.02-0.58]). Although the frequency of tachysytolic pat­terns observed was different between the two groups, (35% preeclamptic vs 16% non-preeclamptic) this difference was not stati­stically significant (p=0.082). It is important to note that an increase of only one case having tachysystole in the preeclamptic group would have converted this difference to significant. Conclusion: Relatively hard induction of labor in preeclamptic patients leads to an increase in the amount of misoprostol used and hence to an increased incidence of tachysystole secondary to the cumulative effect of the drug. These findings seem to support our hypothesis and prove the presence of the cumulative effect of misoprostol.