Background: Colorectal high-grade neuroendocrine carcinomas (HGNEC) are aggressive tumors; treatment options consist of cis/carboplatin and 5FU-based chemotherapy. To date, choice of systemic therapy and sequencing of these drugs remains poorly understood. We examined clinical and molecular characteristics of colorectal HGNEC to better define predictors of response to cis/carboplatin and 5FU-based treatment. Methods: Patients (pts) with colorectal HGNEC treated at MSKCC from 1990-2018 were identified. MANEC (mixed adeno-neuroendocrine carcinoma) were excluded. Demographics, response to first- and second-line chemotherapy (by radiology report), outcomes, and molecular data (next-generation sequencing of tumor tissue), were collected. Results: 65 pts (mean age 58, 52% male) were identified, 13 (20%) with small cell carcinomas. 52 (79%) metastatic, 13 (20%) locally advanced. 27 (42%) received surgery and 11 (17%) received radiation. 56 pts received cis/carboplatin-based therapy, partial response (PR) in 18 (32%), stable disease (SD) in 4 (7%), and progressive disease (PD) in 31 (55%); 3 (5%) did not tolerate therapy. 28 pts received 5FU-based therapy, 13 PR in (46%), SD in 6 (21%), and PD in 7 (25%); 2 (7%) did not tolerate therapy. Median overall survival was 11.4 months. 21/65 (32%) pts underwent molecular sequencing of tumor; the most common alterations were KRAS 11 (52%), TP53 13 (62%), BRAF 7 (33%), APC 8 (38%), RB1 7 (33%). Most tumors (13/21, 62%) harbored alterations in genes traditionally altered in colorectal adenocarcinoma (KRAS/BRAF/APC) and in HGNEC (TP53/RB1). There was no significant difference in response to cis/carboplatin or 5FU-based chemotherapy based on location of the primary tumor (right vs. left) (p = 0.69/0.85), histologic features of the disease (p = 0.71/0.87), and for response to cis/carboplatin by molecular alterations in KRAS (p = 0.94), BRAF (p = 0.24), APC (p = 0.28), TP53 (p = 0.58), or RB1 (p = 0.28). Conclusions: Colorectal HGNEC are highly aggressive and more effective therapies are desperately needed. In this series, OS was poor. Clinical and molecular characteristics failed to predict response to cis/carboplatin and 5FU-based chemotherapy.