Fifty women with previous diagnosis of inadequate luteal phase and threatened abortion underwent a prospective, randomized, double-blind study in one medical center carried out with a parallel trial. The primary objective was to establish the effects of vaginal progesterone (Crinone 8%) in reducing both pain and uterine contractions (UCs). The gel with or without (placebo) vaginal progesterone was administered once a day since the diagnosis of threatened abortion and for 5 days. The efficacy on pain symptom amelioration was evaluated by a 5-score intensity gradation, while the UCs were evaluated by ultrasound. The secondary objective of the study was to evaluate the outcome of the pregnancies. The use of progesterone was effective both on pain relief and on the frequency of the UCs that decreased after 5 days of vaginal progesterone administration (P < 0.005). The evaluation of the ongoing pregnancy and spontaneous abortion in both study groups after 60 days showed that 4 patients of group A and 8 patients of group B miscarried (P < 0.05). In conclusion, patients with threatened abortion benefit from vaginal progesterone by a reduction of UCs and pain. The use of vaginal progesterone improved the outcome of pregnancies complicated by threatened abortion and previous diagnosis of inadequate luteal phase.