Objective: To evaluate seminal oxidative stress in men after vasectomy reversal and to determine whether seminal oxidative stress could predict fertility after vasectomy reversal. Design: Measurement of seminal reactive oxygen species (ROS) and total antioxidant capacity (TAC) in normal donors, men who were fertile after vasectomy reversal, and men who were infertile after vasectomy reversal. Setting: A male infertility clinic of a tertiary care center. Patient(s): Thirty men who underwent vasectomy reversal and 17 normal donors. Intervention(s): None. Main Outcome Measure(s): Semen characteristics, seminal ROS, and TAC were measured with chemiluminescence assays in samples from donors and reversal patients. Result(s): Mean adjusted seminal ROS (log [ROS + 1]) was higher in infertile reversal patients (2.38 ± 0.25) than in normal donors (1.30 ± 0.14). Seminal ROS was also higher in all (fertile and infertile reversal combined) reversal patients than in donors. Total antioxidant capacity did not differ between groups. The ROS- TAC score, a composite index of seminal oxidative stress, was a significant predictor of fertility. A ROS-TAC score of 45 or greater had a positive predictive value of 73% in predicting fertility. Conclusion(s): Seminal oxidative stress is associated with vasectomy reversal. The ROS-TAC score is a possible predictor of infertility after vasectomy reversal.