INTRODUCTION: The study aimed to detect factors predicting a successful pharmacological induction of first-trimester miscarriage. MATERIAL AND METHODS: A prospective, cohort research was conducted at the 1st Department of Obstetrics and Gynecology, Medical University of Warsaw, between years 2011-2015. 642 women diagnosed with first trimester miscarriage qualified for pharmacological induction with misoprostol were included in the study. Each patient underwent repeatable doses of 800mcg misoprostol vaginally. The endpoint was complete excretion of all tissues, with no need to perform surgical curettage. Type of miscarriage, gravidity, parity, number of doses, time of drug administration and side effects were analysed as possible factors influencing the results. The statistical analysis was performed with STATISTICA 10.0 software. RESULTS: The percentage of successful miscarriage induction was 83.6%. Two main factors corresponded with successful pharmacological treatment in regression analysis: number of administered doses (adjusted OR 1.64; 95% CI 1.18-2.29) and week of gestation (aOR 1.22; 95%CI 1.03-1.44). The success of the pharmacological induction of miscarriage was significantly decreased if the woman had a history of caesarean section (aOR 0.34; 95% CI 0.2-0.57). 2.2% of patients experienced benign side effects of the therapy. CONCLUSIONS: Pharmacological induction is an effective and safe treatment method of first trimester abortion in the majority of cases. The knowledge of factors influencing the efficacy of misoprostol may help clinicians in proper counselling and individualisation of therapy.