Abstract Introduction There has been significant ongoing debate on the role and utility of prednisone post-vasectomy reversal to improve sperm counts and long-term patency. However, there are relatively limited data available on the efficacy, with essentially no data available on pregnancy outcomes. Objective To report outcomes of randomized, clinical trial evaluating varying protocols for prednisone administration post vasectomy reversal. Methods A randomized, controlled trial was performed (NCT04788823) with an anticipated 100 men randomized to one of four prednisone protocols post vasectomy reversal: no treatment (Control), prednisone 20 mg taper over 1 month beginning the first month post-op and then every other month x 6 months (High Dose), prednisone 20 mg taper over 1 month if declining sperm counts or a 0 count were identified (PRN), and prednisone 5 mg daily x 1 week, followed by no treatment x 1 week, continued for 6 months (Low Dose). Randomization tables were created a priori to ensure an equal representation of differing durations of time since the original vasectomy as well as an equal distribution of VV/VV vs VV/EV vs EV/EV procedures. All men had semen analyses obtained monthly for a first 12 months. Surveys were sent periodically to follow-up on subsequent pregnancies. Inclusion criteria included having a female partner <36 years of age with no known fertility factors. Results A total of 75 men were ultimately enrolled due to interim findings suggesting inferiority of the High Dose and PRN treatment arms and decision to close those arms before completing full enrollment. Final enrollment included: Control (n=25), High Dose (n=14), PRN (n=11), and Low Dose (n=25). The mean patient age was 38.5 years, mean partner age 29.6, and mean time since vasectomy 6.3 years (SD 5.0). 82% of men underwent VV/VV, while 15% VV/EV, and 3% EV/EV. All men underwent successful anastomosis bilaterally with no cases of one-sided or no anastomosis performed. At a mean follow-up of 16.5 months (range 7.6 to 25.1), an overall patency (sperm observed on sample) rate of 93% was noted (Control 96%, High Dose 93%, PRN 100%, Low Dose 88%, p=0.41). The mean sperm concentration post-op was 34.4 million/ml (Control 24.0, High Dose 38.4, PRN 21.0, Low Dose 49.8, p=0.33). Patency rates at 6- and 12-months were: 6-months (Control 88%, High Dose 80%, PRN 83%, Low Dose 94%, p=0.75); 12-months (Control 75%, High Dose 71%, PRN 75%, Low Dose 91%). Mean sperm concentrations at 6- and 12-months were: 6-months (Control 15.7, High Dose 21.5, PRN 6.9, Low Dose 25.7, p=0.50); 12-months (Control 12.6 million/ml, High Dose 17.1, PRN 36.1, Low Dose 50.4, p=0.64) At the 6-month post-op time point, the overall rate of pregnancy was 47%, with notable differences between cohorts: Control 57%, High Dose 10%, PRN 38%, and Low Dose 67%, p=0.02. No patients discontinued prednisone due to side effects. Conclusions Prednisone taper protocols appear to reduce the likelihood for pregnancy following vasectomy reversal. Use of low-dose prednisone results in non-statistically higher rates of patency at 6- and 12-months, mean sperm concentration at 6- and 12-months, and pregnancy at 6-months. Overall results suggest superiority of either no prednisone post-vasectomy reversal or a low dose protocol but argue against the use of higher dosages at any time due to its negative impacts on pregnancy rates despite maintained sperm counts. Disclosure No.