OBJECTIVE: To compare the efficacy and the tolerability of two different vaginal formulations of micronized progesterone, Ellios and Utrogestan, used for luteal phase support after an in vitro fertilization (IVF) cycle. DESIGN: Cohort study. SETTING: Fertility center in a university hospital. PATIENT(S(: One hundred twenty-three women who underwent IVF/intracytoplasmic sperm injection (ICSI) stimulated cycles from October 1998 to March 2000, who had at least six follicles of > or =14 mm on the day of hCG administration. INTERVENTION(S): Patients received Ellios pessaries (2 times 200-mg pessary/day) or Utrogestan capsules (2 x 100-mg capsules, two times a day). Progesterone was administered from the day of oocyte pickup (day 0) until menses or up to 10 weeks in pregnant patients. MAIN OUTCOME MEASURE(S): Progesterone levels, pregnancy rate, and tolerability tested by patient questionnaire. RESULT(S): The progesterone levels on days 0, 9, 16 were not statistically different between the two formulations. The pregnancy rate were similar in groups 1 and 2 (25.5% vs. 18.6%), whereas tolerance was significantly better in group 1 versus group 2 (vaginal discharge: 43% vs. 82%). CONCLUSION(S): The efficacy of the two formulations of progesterone is comparable, although the patient tolerance for Ellios is better.