Objective: To investigate the efficacy and safety of intravaginal Crinone 8% (Columbia Research Laboratories, Miami, FL) versus LM progesterone for luteal phase support after IVF-ET. Design: Prospective open trial with comparison to historical controls. Setting: University hospital. Patient(s): Two hundred six women undergoing IVF-ET. Intervention(s): One hundred patients received Crinone vaginal progesterone gel (90 mg once daily) and 106 patients received IM progesterone (50 mg once daily) beginning on the evening of oocyte retrieval. Main Outcome Measure(s): Pregnancy and miscarriage rates, and midluteal serum progesterone levels. Result(s): Positive beta-hCG pregnancy rates, clinical pregnancy rates per transfer, and ongoing pregnancy rates were similar for the Crinone and LM progesterone groups. Women who received Crinone had higher rates of biochemical pregnancy loss but lower rates of clinical pregnancy loss (i.e., spontaneous abortion) than women who received TM progesterone. Midluteal serum progesterone concentrations were significantly higher in the IM progesterone group (94.3 +/- 8.8 ng/mL versus 57.7 +/- 7.4 ng/mL). Several women who received Crinone had vaginal bleeding 11-13 days after oocyte retrieval. Conclusion(s): For all age categories, positive beta-hCG and ongoing pregnancy rates were similar when Crinone or IM progesterone was given for luteal phase support in IVF-ET cycles, despite lower serum progesterone concentrations and higher rates of biochemical pregnancy loss with Crinone. Although the results of this study support the use of Crinone as an acceptable alternative for luteal support after IVF-ET, differences in bleeding patterns and rates of biochemical pregnancy loss demonstrate the need for a prospective randomized study. (Fertil Steril(R) 1999;72:823-9. (C) 1999 by American Society for Reproductive Medicine.).