The ability of sildenafil to aid in the return of erections after nerve-sparing radical prostatectomy has been established. Patients who had either one or both neurovascular bundles spared demonstrated dramatically better responses to "as needed" sildenafil than those that did not, and a positive erectile response to sildenafil was only seen in patients in whom at least one NVB was spared. This study has been designed to determine if sildenafil taken nightly works better than sildenafil on as "as needed" basis for the return of erectile function. The investigators hypothesis is that sildenafil taken nightly promotes a more rapid return of erectile function after nerve-sparing laparoscopic radical prostatectomy.

Inclusion Criteria:

1. Male sex
2. Age < 65
3. IIEF erectile function domain score > 26 (out of 30 points possible for this subscale)
4. Steady sexual partner
5. Untreated prostate cancer TNM stage < cT2bNxMx (cT1a, cT1b, cT1c, cT2a) and Gleason grade < 8.
6. Willingness to participate in a clinical trial as manifested by informed consent
7. Actually undergo nerve-sparing LRP surgery

Exclusion Criteria:

1. Not fulfilling all of the criteria for entry above
2. Any prior prostate cancer treatment (radiation, hormonal deprivation, chemotherapy)
3. Contraindication to sildenafil (e.g. nitrates, hypersensitivity)
4. Existing PDE5 inhibitor requirement for functional erection (e.g. for intercourse) preoperatively
5. Obstructive sleep apnea