Purpose/Objective(s) The use of radiation therapy (RT) and androgen deprivation therapy (ADT) following radical prostatectomy (RP) has been recently addressed by the NRG/RTOG 0534 phase III trial, which compared prostate bed RT alone (arm 1), prostate bed RT + short term (st-ADT) (arm 2), and prostate bed RT + st-ADT + pelvic lymph node (LN) RT (arm 3). A wide range of LN volumes were removed at RP prior to enrollment. We sought to study the influence of LN dissection volumes on clinical outcomes. Materials/Methods Our hypothesis was that the number of LNs resected at RP on NRG/RTOG 0534 influences the benefit of pelvic LN RT (arm 3). Outcomes of interest included freedom from progression (FFP), time to distant metastasis (DM) and toxicity. Pretreatment characteristics were compared across quartiles of number of LNs dissected using the Kruskal Wallis test for categorical variables and the f-test from analysis of variance for continuous variables. Adjusted multivariable (MV) Cox proportional hazards models (cause-specific in the presence of competing risks) were used to assess the interaction between treatment arm and LNs dissected (0-1 vs. 2+ and as a continuous variable) with FFP and time to DM. Results 1,626 eligible patients had data on number of LNs dissected, 538 in arm 1, 543 in arm 2, and 545 in arm 3. 606 patients had 0 LNs removed, 228 had 1-2, 414 had 3-7, and 378 had over 7 LNs removed (max of 54). A higher number of LNs removed was associated with more adverse Gleason grade (p< 0.01), T-stage (p< 0.01), seminal vesicle invasion (p < 0.01), and academic medical center enrollment (p < 0.01). Controlling for differences in patient characteristics and continuous LN dissected, patients on arm 3 (vs. arm 2) did not demonstrate improvement in FFP (HR 1.19, 95% CI: 0.93-1.51, p = 0.2) nor was the interaction between treatment and LN dissected significant. Upon MV analysis for DM, there was a significant interaction between continuous number of LNs dissected and treatment arm. Specifically, patients with 0 LNs removed on arm 3 had a significant improvement in DM when compared with arms 1 and 2 (vs. arm 1 HR 2.49, 95% CI: 1.46-4.26; vs. arm 2 HR 1.75, 95% CI: 1.02-3.01). Patients with 1 node removed and on arm 3 had a significant improvement in DM when compared with arms 1 and 2 (vs. arm 1 HR 2.34, 95% CI: 1.42-3.85; vs. arm 2 HR 1.67, 95% CI: 1.00-2.77). The benefit of arm 3 vs. arm 2 was no longer significant in patients with 2+ LNs removed as the 95% CIs crossed 1.0. 2+ vs. 0-1 pelvic LNs removed and treatment on arm 3 vs. 1 exhibited higher rates of any grade renal/genitourinary toxicity (HR=1.16, 95% CI:1.02-1.31; HR=0.86, 95% CI: 0.75-0.99, respectively). Conclusion The extent of surgical pelvic LN dissections influences the benefit of post-operative pelvic LN RT on FFP and DM. Patients with 0-1 LNs removed, and randomized to pelvic LN RT and st-ADT, benefitted significantly from the addition of pelvic LN RT. Although no improvements with pelvic LN RT were seen in patients with 2+ nodes removed, these analyses had low statistical power.