The ubiquitin–proteasome pathway regulates bone formation through osteoblast differentiation. We analyzed variation alkaline phosphatase (ALP) during carfilzomib treatment. Data from 38 patients enrolled in the PX-171-003 and 29 patients in PX-171-004 studies, for patients with relapsed/refractory myeloma, were analyzed. All patients received 20 mg/m2 of carfilzomib on Days 1, 2, 8, 9, 15, and 16 of a 28-day cycle. Sixty-seven patients from ALP data were evaluable. In PX-171-003, the ORR (>PR) was 18% and the clinical benefit response (CBR; >MR) was 26%, while in PX-171-004, the ORR was 35.5% overall and 57% in bortezomib-naive patients. ALP increment from baseline was statistically different in patients who achieved ≥VGPR compared with all others on Days 1 (P = 0.0049) and 8 (P = 0.006) of Cycle 2. In patients achieving a VGPR or better, ALP increased more than 15 units per liter at Cycle 2 Day 1 over baseline. An ALP increase over the same period of time was seen in 26%, 13% and 11% of patients achieving PR, MR, and SD, respectively. This retrospective analysis of patients with relapsed or refractory myeloma treated with single-agent carfilzomib indicates that early elevation in ALP is associated with subsequent myeloma response.