OBJECTIVE To determine whether the period of ejaculatory abstinence (EA) influences the total antioxidant capacity (TAC) of semen or lipid peroxidation (LPO) of sperm membranes. DESIGN A prospective experimental trial. SETTING Academic medical center for reproductive endocrinology and infertility. PATIENT(S) Forty men from infertile couples planning intrauterine insemination. INTERVENTION(S) Men provided semen specimens after EA periods of 1 and 4 days. MAIN OUTCOME MEASURE(S) Semen analysis, peroxidase staining, and assays for seminal TAC and sperm membrane LPO, with measures compared between days 1 and 4 within individuals (internal control) using paired t tests. RESULT(S) The shorter period of EA (1 day vs. 4 days) resulted in statistically significant decreases in semen volume (-24%), sperm density (-28%), and total sperm count (-3.2%). There was a statistically significant increase in TAC with the shorter period of EA (1 day) compared with 4 days of EA. No difference was detected in sperm membrane LPO comparing 1 day of EA and 4 days of EA. CONCLUSION(S) Higher seminal TAC obtained after a shorter period of EA could diminish oxidative stress-induced sperm damage by a mechanism independent of LPO. Shorter periods of EA may thus improve sperm quality by protecting from reactive oxygen species damage, even though lower numbers of motile sperm are produced after a shorter period of EA. This would be consistent with prior research indicating improved results after intrauterine insemination under these circumstances.