Background: Patients (pts) with rare cancers represent an unmet medical need and have an inferior overall survival compared to patients with more common malignancies. Due to their low frequency, no therapies, including immunotherapies, have systematically been investigated in this population. Ipilimumab (ipi)/Nivolumab (nivo) combination treatment has demonstrated significant clinical activity in pts with advanced melanoma and renal cell carcinoma and response rates with this regimen are higher compared to single agent anti-PD-1 therapy. This phase II study assessed the efficacy and safety of ipi/nivo in rare cancer pts. Methods: 60 pts with advanced rare upper gastrointestinal (GI), neuroendocrine (NE) and gynaecological (GY) malignancies were enrolled in 3 cohorts. Patients received nivo 3mg/kg and ipi 1mg/kg every 3 weeks for four doses, followed by nivo 3mg/kg every 2 weeks. Treatment continued for up to 96 weeks, or until disease progression or the development of unacceptable toxicity. Response (RECIST 1.1) was assessed every 12 weeks. The primary endpoint was clinical benefit rate (CBR), CR, PR and SD. Exploratory endpoints include correlation of efficacy with relevant biomarkers including PDL1 status and tumour mutation burden. Results: 42 pts have so far undergone restaging, 11 pts clinically progressed prior to their first restaging scan. 50 pts have received prior therapy (1-5 lines). Objective responses have been observed in a range of different malignancies. Clinical trial information: NCT02923934 Grade 3/4 immune related adverse events were detected in 31% of pts. The results of correlative biomarker studies will be presented at the meeting. Conclusions: Ipi/Nivo combination treatment has efficacy in a wide range of advanced rare malignancies. Immune related toxicity is in keeping with previously reported clinical trials using the same dosing regimen.