Background: In phase 1 of the KEYNOTE-051 study (NCT02332668), the 2 mg/kg Q3W approved adult dose of pembro was also determined to be the pediatric recommended phase 2 dose. In phase 2, presented herein, we further evaluated this dose in pediatric pts with advanced cancer. Methods: Children aged 6 mo to < 18 y with advanced melanoma or a PD-L1⁺ advanced, relapsed or refractory solid tumor or lymphoma, measurable disease per RECIST v1.1, and performance score ≥50 using Lansky Play or Karnofsky scales received pembro 2 mg/kg Q3W for 35 cycles or until confirmed disease progression per immune-related RECIST by investigator review, intolerable toxicity, or pt/investigator decision to discontinue. Tumor imaging was performed every 8 wk for the first 6 mo, then every 12 wk thereafter. AEs were graded by NCI CTCAE v4.0. Key efficacy endpoints were ORR, disease control rate (DCR), and PFS per RECIST v1.1 by investigator and OS. Results: Of 369 pts prescreened, 364 were evaluable for PD-L1 expression; of these, 121 (33.2%) were PD-L1⁺. 66 pts were enrolled; median follow-up was 2.5 mo (range, 0.2-18). As of the data cutoff (Nov 7, 2016), 23 (34.8%) pts were still on treatment. Median age was 13 y (range, 1-17), 77.3% had metastatic disease, and 34.8% had ≥3 prior lines of therapy for recurrent/metastatic disease. Primary diagnoses were non-CNS solid tumors (n = 45), CNS tumors (n = 16), and lymphoma (n = 5). 5 (7.6%) pts had grade 3-4 treatment-related AEs (TRAEs), most commonly neutropenia (n = 2). No treatment-related deaths occurred; 1 pt discontinued for a TRAE (grade 3 AST increased). 1 pt each with Hodgkin lymphoma, adrenocortical carcinoma, mesothelioma, and glioblastoma had partial response for an ORR of 6.1% (95% CI, 1.7-14.8); 7 (10.6%) pts had stable disease for a DCR of 16.7% (95% CI, 8.6-27.9). Median PFS and OS were 1.8 mo and 9.2 mo, respectively; 12-mo PFS was 10.2% and OS was 40.5%. Potential effects of pembro on the developing immune system (eg, T and B cells, vaccinated antibodies) will also be presented. Conclusions: Pembro showed low toxicity and warrants further study to determine activity in select pediatric tumors. Enrollment in KEYNOTE-051 is ongoing. Clinical trial information: NCT02332668