Estrogen (E₂) and plasma progesterone (P₄) levels are valuable parameters for follicular development in in vitro fertilization (IVF) cycles. Furthermore, the progesterone concentration prior to, during, and following human chorionic gonadotropin (hCG) administration is an important marker for the detection of early luteinization and premature ovulation. The pattern of hormonal profile in relation to the number of oocytes retrieved, fertilized, and cleaved and the fate of the pregnancies achieved were compared in three groups of patients treated by the same protocol. Group I included 22 women who conceived with high progesterone levels on day hCG+1 (P_4'>2.5 ng/ml). Group II included 43 women who conceived with low P₄ values (P_4'<2.5 ng/ml),="" while="" group="" iii="" included="" 46="" patients="" in="" whom="" no="" pregnancies="" occurred.="" a="" significant="" decrease="" in="" fertilization,="" cleavage,="" and="" pregnancy="" rates="" was="" observed="" in="" patients="" with="" high="" progesterone="" levels="" on="" day="" hcg+1,="" compared="" to="" those="" with="" normal="" levels.="" nevertheless,="" it="" is="" suggested="" that="" cycles="" with="" high="">₄ levels in the preovulatory phase should not be canceled, as a fair chance for pregnancy still exists.