Aims: To explore the biomarker for predicting the occurrence of adverse events in myeloma patients treated by intravenous bortezomib, we measured proteasome activity in peripheral blood mononuclear cells. Methods: Samples were obtained from 34 bortezomib-naïve patients. Proteasome activity was measured at pre- and postchemotherapy phase by using a synthetic substrate. Results: Bortezomib injection resulted in a dramatic decrease in proteasome activity, reaching 32.4 ± 18.79% (mean ± SD) of the pretreatment level at 1 h, but it generally recovered at the end of the first course. In total, 6 patients manifested with severe bortezomib-induced peripheral neuropathy (sBIPN) in the second-third course. There was a nonsignificant trend for these patients to have lower levels of the relative proteasome activity at the end of the first course than those without sBIPN (median: 74.03 vs. 103.2%, p = 0.052). Moreover, in all of them, proteasome activity did not recover to the pretreatment level, whereas no patients with complete recovery manifested with sBIPN. Analysis with Fisher's exact test demonstrated that incomplete recovery of proteasome activity is a significant risk factor for sBIPN (p = 0.014). Conclusion: Patients with incomplete recovery of proteasome activity are at high risk for developing sBIPN, and the susceptible patients can be indicated by monitoring proteasome activity.