Background: Neoadjuvant pembrolizumab at 200 mg in combination with high dose IFNα2b (HDI) for locally/regionally advanced or recurrent melanoma may improve the clinical outcomes of these high risk patients (pts), and provide access to blood and tumor pre/post pembro-HDI to illuminate the host effector and suppressor immune mechanisms. Methods: Pts were treated with pembro 200 mg IV every 3 weeks (wk) x 2 doses followed by definitive surgery, then every 3 wks for up to one year. HDI (20 MU/m²/d IV x 5 days (d)/wk for 4 wks then 10 MU/m²/d SC every other d TIW for 48 wks) was given concurrently. Tumor samples were obtained at baseline and at definitive surgery (wk 6-8) and serum/PBMC at baseline, 6 wks, 3, 6, 12 months (mo). Results: Twenty evaluable pts (14 male, 6 female, 14 cutaneous primary, 4 unknown, 3 mucosal), age 29-82 were treated. 5 had Stage IIIB (N1b, N2b, M2c), 11 IIIC (N3) and 4 IV (M1a, M1b) melanoma. Over 230 cycles have been delivered to date (median 14). Worst toxicities included grade (Gr) 3: fatigue (8; 40%), ↑CPK (5; 25%), hypophosphatemia (5; 25%), ↑lipase (3; 15%), lymphopenia (3; 15%), hypertension (2; 10%), diarrhea/colitis (1; 5%), arthralgia (1, 5%), syncope (1, 5%), hyponatremia (1, 5%), neutropenia (1; 5%), anemia (1, 5.00%) nausea (2, 10%), flu like symptoms (1, 5%). There were 3 Gr 4 events (CPK, hyperglycemia, lymphopenia). One suspected grade 5 event occurred 6 months after completion of therapy with autopsy evidence of pneumonia and myocarditis. Among 20 evaluable pts, 4 relapsed and 1 died. Median follow-up for pts who have not relapsed is 11 months. The radiologic preoperative response rate (WHO; unconfirmed) was 65%. The pathologic complete response rate (no viable tumor on histologic assessment) was 35%. Conclusions: Neoadjuvant pembro-HDI exhibited promising clinical activity. Longer follow up is underway in order to define the long term benefits and risks. Clinical trial information: NCT02339324