Background Prehabilitation is a promising modality for improving patient-related outcomes after major surgery, however, very little research has been done for those who may need it the most; the elderly and the frail. The study aimed to investigate if high risk, frail patients with colorectal cancer and WHO performance status I and II could adhere to a short course multimodal prehabilitation intervention prior to surgery.Methods The study was conducted as a single-center, prospective one-arm feasibility study with limited-efficacy testing of eight patients. The intervention consisted of a physical training program tailored to the patients with both high-intensity interval training and resistance training three times a week, nutritional support with protein and vitamins, a consultation with a dietician, and medical optimization prior to surgery. The primary outcomes were the increase in maximum oxygen intake (VO2 peak) during the intervention and per-protocol compliance and adherence. Secondary outcomes were: increase in body weight, increase in albumin, adverse events during the intervention, postoperative length of stay, and complications within 30 days after surgery. Results The mean age vas 80 (range: 66-88). Seven completed the intervention. The mean increase in maximum oxygen intake (VO2 peak) was 17 % (range: 0.6% - 28 %), with two patients being non-responders. All patients who had a prediagnostic weight loss increased their body weight with a mean of 2.5 kg (range: 1.3 – 4.0 kg). Adherence to high-intensity interval training was 87 %, whereas adherence to nutritional support was 57 %. The median postoperative length of stay was 3 days (range: 1-26).Conclusions This one-arm feasibility study indicates that multimodal prehabilitation including high-intensity interval training can be performed with high adherence to the program in patients with colorectal cancer and WHO performance status I and II Clinical trial registration Clinicaltrials.gov: NCT04167436, Date of registration November 18 2019. Retrospectively registered.