Aim: 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. Materials and Methods: 560 ICSI cycles performed for male factor infertility were divided into four groups according to sperm concentration retrospectively. Group I consisted of 86 couples whose sperm concentration was less than 1 x 10(6), group 2 consisted of 169 couples whose sperm concentration ranged between 1 x 10(6) and 5 x 106, group 3 consisted of 95 couples whose sperm concentration ranged between 5 x 10(6) and 10 x 10(6) and group 4 consisted of 210 couples whose sperm concentration ranged between 10 x 10(6) and 20 x 10(6), Results: 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. Conclusion: 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.