In this randomized phase 2 study (GEM-KyCyDex), the combination of weekly carfilzomib 70 mg/m2, cyclophosphamide and dexamethasone was compared to carfilzomib and dexamethasone (Kd) in relapsed/refractory multiple myeloma (RRMM) after 1-3 prior lines (PLs). 197 patients were included and randomized 1:1 to receive KCd (97 patients) or Kd (100 patients) in 28-day cycles until progressive disease or unacceptable toxicity occurred. Patient median age was 70 years, and the median number of PLs was 1 (1-3). More than 90% of patients had previously been exposed to proteasome inhibitors, ≈70% to immunomodulators, and ≈50% were refractory to their last line (mainly lenalidomide) in both groups. After a median follow-up of 37 months, median progression-free survival (PFS) was 19.1 and 16.6 months in KCd and Kd, respectively (P=0.577). Of note, in the post hoc analysis of the lenalidomiderefractory population, the addition of cyclophosphamide to Kd resulted in a significant benefit in terms of PFS: 18.4 vs. 11.3 months (Hazard ratio 1.7 1.1-2.7; P=0.043). The overall response rate and the percentage of patients who achieved complete response was around 70% and 20% in both groups. The addition of cyclophosphamide to Kd did not result in any safety signal, except for severe infections (7% vs. 2%). In conclusion, the combination of cyclophosphamide with Kd 70 mg/m2 weekly does not improve outcomes as compared with Kd alone in RRMM after 1-3 PLs, but a significant benefit in PFS was observed with the triplet in the lenalidomide-refractory population. The administration of weekly carfilzomib 70 mg/m2 was safe and convenient, and, overall, the toxicity was manageable in both arms.