The proteasome inhibitors, carfilzomib and bortezomib, are widely used to treat myeloma but head-to-head comparisons have produced conflicting results. We compared the activity of these proteasome inhibitors in combination with cyclophosphamide and dexamethasone (KCd vs. VCd) in second-line treatment using fixed duration therapy and evaluated the efficacy of carfilzomib maintenance. MUKfive was a phase II controlled, parallel group trial that randomized patients (2:1) to KCd (n=201) or VCd (n=99); responding patients on carfilzomib were randomized to maintenance carfilzomib (n=69) or no further treatment (n=72). Primary endpoints were: (i) very good partial response (non-inferiority, odds ratio [OR] 0.8) at 24 weeks, and (ii) progression-free survival. More participants achieved a very good partial response or better with carfilzomib than with bortezomib (40.2% vs. 31.9%, OR=1.48, 90% confidence interval [CI]: 0.95, 2.31; non-inferior), with a trend for particular benefit in patients with adverse-risk disease. KCd was associated with higher overall response (partial response or better, 84.0% vs. 68.1%, OR=2.72, 90% CI: 1.62, 4.55, P=0.001). Neuropathy (grade ≥3 or ≥2 with pain) was more common with bortezomib (19.8% vs. 1.5%, P<0.0001), while grade ≥3 cardiac events and hypertension were only reported in the KCd arm (3.6% each). The median progression-free survival in the KCd arm was 11.7 months vs. 10.2 months in the VCd arm (hazard ratio [HR]=0.95, 80% CI: 0.77, 1.18). Carfilzomib maintenance was associated with longer progression-free survival, median 11.9 months vs. 5.6 months for no maintenance (HR 0.59, 80% CI: 0.46-0.77, P=0.0086). When used as fixed duration therapy in first relapase, KCd is at least as effective as VCd, and carfilzomib is an effective maintenance agent. This trial was registered with International Standard Randomised Controlled Trial Number (ISRCTN) identifier: ISRCTN17354232.