Background: Antitumor activity with pembro, an anti–PD-1 antibody, has been observed in patients (pts) with advanced/metastatic biliary tract cancers (BTC), who have limited treatment options. We present follow-up data from pts with advanced BTC treated with pembro in the KN158 (NCT02628067; phase 2) and KN028 (NCT02054806; phase 1) studies. Methods: Eligible pts ≥18 y in the KN158/KN028 BTC cohorts had histologically/cytologically confirmed incurable advanced BTC that progressed after/failed any number of prior standard treatment regimens, measurable disease per RECIST v1.1, ECOG PS of 0/1, and no prior immunotherapy. PD-L1–positivity (membranous PD-L1 expression in ≥1% of tumor and associated inflammatory cells or positive staining in stroma) was required for eligibility in KN028, but not KN158. Pts received pembro 200 mg Q3W (KN158) or 10 mg/kg Q2W (KN028) for up to 2 y. Radiographic imaging occurred Q9W for 12 mo (KN158) or Q8W for 6 mo (KN028) and Q12W thereafter. Primary efficacy endpoint in both studies was ORR by RECIST 1.1. Response assessed by independent central review is reported. Results: Median (range) follow-up was 7.5 (0.6–29.5) mo in the 104 pts from KN158 and 6.5 (0.6–33.1) mo in the 24 pts from KN028 with BTC. All pts in KN028 and 61 in KN158 had PD-L1–positive tumors. No pt had MSI-H tumors (not assessed in KN028). In KN158, ORR was 5.8% (6/104, all PR [including 1 pt with PD-L1–negative tumor]; 95% CI, 2.1%–12.1%) and median duration of response (DOR) was not reached (NR; range, 6.2 to 23.2+ mo). Median OS and PFS were 7.4 mo (95% CI, 5.5–9.6) and 2.0 mo (95% CI, 1.9–2.1). 12-mo OS rate was 32.7%. In KN028, ORR was 13.0% (3/23, all PR; 95% CI, 2.8%‒33.6%) and median DOR was NR (range, 21.5 to 29.4+ mo). Median OS and PFS were 6.2 mo (95% CI, 3.8‒10.3) and 1.8 mo (95% CI, 1.4‒3.7), respectively. 12-mo OS rate was 27.6%. Grade 3–5 treatment-related AEs occurred in 13.5% in KN158 (1 pt had grade 5 renal failure) and 16.7% of pts in KN028 (no grade 5). 18.3% in KN158 and 20.8% of pts in KN028 had an immune-mediated AE or infusion reaction. Conclusions: Pembro provides durable antitumor activity, regardless of PD-L1 expression, and manageable toxicity in a subset of pts with advanced BTC. Clinical trial information: NCT02054806 and NCT02628067