The present study was conducted to evaluate the influence of sperm:oocyte ratio during in vitro fertilization (IVF) of in vitro matured cumulus-intact oocytes on fertilization parameters and embryo development in pigs. In vitro matured oocytes surrounded by intact cumulus cells (COC) were inseminated with frozen-thawed spermatozoa at different sperm:oocyte ratios (2000:1, 3000:1, 4000:1, 6000:1, and 8000:1). Denuded oocytes inseminated with 2000 frozen-thawed spermatozoa:oocyte were the control group. A total of 2546 oocytes in five replicates were exposed to spermatozoa for 6 h and then cultured in embryo culture (EC) medium for 6 h (pronuclear formation) or 7 days (blastocyst formation: BF). The penetration rate increased in the COC groups with the sperm:oocyte ratio, reaching the highest rates with 8000:1 spermatozoa:oocyte (72.1 +/- 6.5%), similar to the control (73.5 +/- 3.5%). However, the monospermy was highest with the lower spermatozoa:oocyte rates (82.6-94.8%) and decreased drastically (P<0.05) in the COC group fertilized with 8000 sperm:oocyte (36%). The efficiency of fertilization (number of monospermic oocytes/total number of inseminated oocytes) showed no difference among the COC groups (20-30%) but they were significantly lower (P<0.007) than those obtained by the control group (43.7 +/- 2%). Embryo development was highest in the control group (58% for cleavage and 23% for BF) but not significantly different with the 6000 and 8000 sperm:oocyte COC groups (47 and 50% for cleavage and 19 and 17% for BF, respectively). These results indicate that the use of COC for IVF involves a drop in the efficiency of the fertilization and the necessity to increase the frozen-thawed sperm:oocyte ratio three to four times more to obtain similar embryo development to denuded oocytes.