The aim of this study was to evaluate the sole effect of sperm concentration on fertilization, embryo quality and pregnancy rates in patients undergoing ICSI cycles.560 ICSI cycles performed for male factor infertility were divided into four groups according to sperm concentration retrospectively. Group 1 consisted of 86 couples whose sperm concentration was less than 1x10(6), group 2 consisted of 169 couples whose sperm concentration ranged between 1x10(6) and 5x10(6), group 3 consisted of 95 couples whose sperm concentration ranged between 5x10(6) and 10x10(6) and group 4 consisted of 210 couples whose sperm concentration ranged between 10x10(6) and 20x10(6).Fertilization rate was significantly lower in the first three groups compared to the last group (p<0.05). The first three groups were comparable with each other. There were no differences according to ovarian response to stimulation, embryo quality and clinical pregnancy rates between the four groups.Lower sperm concentration has detrimental effects on the outcomes of ICSI cycles. This situation is more evident in men with severe and extremely severe oligozoospermia.