To determine the optimal interval of ejaculatory abstinence for couples undergoing IUI. Design: Retrospective analysis. Setting: Reproductive endocrinology and infertility center. Patient(s): Infertile couples undergoing ovulation induction and IUI with partner’ s semen. Intervention(s): Ovulation induction with clomiphene citrate and a single IUI procedure per cycle. Main Outcome Measures(s): Clinical pregnancy rates as a function of abstinence intervals. Result(s): Four hundred seventeen women underwent 929 cycles from June 1999 to October 2002 for a median of 4 IUI attempts per couple. The median ejaculatory abstinence interval was 4 days (range 0- 30) with an overall pregnancy rate of 12% per cycle. Abstinence correlated positively with inseminate sperm count but negatively with motility. Variations in inseminate parameters did not correlate with pregnancy rates. However, abstinence intervals significantly affected pregnancy rates. The highest pregnancy rate was observed with an abstinence interval of 3 days or less (14% ) and the lowest pregnancy rate seen with an abstinence interval of 10 days or more (3% ). Conclusion(s): An abstinence interval of 3 days or less was associated with higher pregnancy rates following IUI. Prolonged abstinence decreases pregnancy rates, independent of other sperm parameters, perhaps as a result of sperm senescence and functional damage not readily identified by standard semen analysis. Abstinence intervals should be controlled for in studies examining pregnancy outcome in assisted reproduction. To determine the optimal interval of ejaculatory abstinence for couples undergoing IUI. Design: Retrospective analysis. Setting: Reproductive endocrinology and infertility center. Patient (s): Infertile couples undergoing ovulation induction and IUI with partner ’s semen. Intervention (s): Ovulation Indices with clomiphene citrate and a single IUI procedure per cycle. Main Outcome Measures (s): Clinical pregnancy rates as a function of abstinence intervals. (s): Four hundred seventeen women underwent 929 cycles from June 1999 to October 2002 for a median of 4 IUI attempts per couple. The median ejaculatory abstinence interval was 4 days (range 0- 30) with an overall pregnancy rate of 12% per cycle. Abstinence correlated positively with inseminate sperm count but negatively with motility. Variations in inseminate parameters did not However, abstinence intervals significantly affected pregnancy rates. The highest pregnancy rate was observed with an ab stinence interval of 3 days or less (14%) and the bottom pregnancy rate seen with an abstinence interval of 10 days or more (3%). Conclusion (s): An abstinence interval of 3 days or less was associated with higher pregnancy rates following IUI. Prolonged abstinence decreases pregnancy rates, independent of other sperm parameters, perhaps as a result of sperm senescence and functional damage not determined by standard semen analysis. Abstinence intervals should be controlled for in studies examining pregnancy outcome in assisted reproduction.