['To evaluate the impact of sperm defect severity and the type of azoospermia on the outcomes of intracytoplasmic sperm injection (ICSI).', 'This study included 313 ICSI cycles that were divided into two major groups according to the source of spermatozoa used for ICSI: 1) Ejaculated (group 1; n = 220) and 2) Testicular/Epididymal (group 2; n = 93). Group 1 was subdivided into four subgroups according to the results of the semen analysis: 1) single defect (oligo-[O] or astheno-[A] or teratozoospermia-[T], n = 41), 2) double defect (a combination of two single defects, n = 45), 3) triple defect (OAT, n = 48), and 4) control (no sperm defects; n = 86). Group 2 was subdivided according to the type of azoospermia: 1) obstructive (OA: n = 39) and 2) non-obstructive (NOA: n = 54). Fertilization (2PN), cleavage, embryo quality, clinical pregnancy and miscarriage rates were statistically compared using one-way ANOVA and Chi-square analyses.', 'Significantly lower fertilization rates were obtained when either ejaculated sperm with triple defect or testicular sperm from NOA patients (63.4 +/- 25.9% and 52.2 +/- 29.3%, respectively) were used for ICSI as compared to other groups ( approximately 73%; P < 0.05). Epididymal and testicular spermatozoa from OA patients fertilized as well as normal or mild/moderate deficient ejaculated sperm. Cleavage, embryo quality, pregnancy and miscarriage rates did not differ statistically between ejaculated and obstructive azoospermia groups. However, fertilization, cleavage and pregnancy rates were significantly lower for NOA patients.'] Is sperm defect severity rather than sperm Source associated with lower fertilization rates after intracytoplasmic sperm injection?