This multicentre, open-label phase 1/2 trial determined safety and efficacy of weekly carfilzomib plus cyclophosphamidedexamethasone (wKCyd) in newly diagnosed multiple myeloma (NDMM) patients aged ≥ 65 years or transplant ineligible. Patients received wKCyd for up to nine 28-day cycles, followed by maintenance with carfilzomib until progression/intolerance. The phase 1 portion used a 3+3 dose-escalation scheme to determine the maximum tolerated dose of weekly carfilzomib: 12 patients received wKCyd with carfilzomib doses of 45, 56 and 70 mg/m2. The recommended phase 2 dose was established at 70 mg/m2 and 54 patients (phase 1 and 2) received weekly carfilzomib 70 mg/m2: 85% of them achieved ≥ partial response (PR), 66% ≥ very good PR, 30% ≥ near-complete response (CR) and 15% CR. Responses improved in 40 patients who started maintenance: 98% achieved ≥ PR, including 29% CR and 10% stringent CR. After a median follow-up of 18 months, the 2-year progression-free survival and overall survival rates were 53.2% and 81%, respectively. The most frequent grade 3-5 toxicities were neutropenia (22%) and cardiopulmonary adverse events (9%). This is the first study of weekly carfilzomib plus an alkylating agent in elderly patients with NDMM. wKCyd was effective, with an acceptable risk/benefit ratio, and thus can be a valid option in this setting.