Background: Cabazitaxel (Cbz) is approved in combination with prednisone/prednisolone for the treatment of men with hormone-refractory metastatic prostate cancer previously treated with a docetaxel-containing treatment regimen. In vitro studies showed that Cbz is mainly metabolized through CYP3A, resulting in inhibition of this family of enzymes. Midazolam (Mdz) is primarily metabolized by CYP3A4. We aimed to determine the effect of Cbz on CYP3A activity by comparing the pharmacokinetic (PK) properties of Mdz when administered alone and following co-administration with Cbz. Methods: An ongoing safety and PK study of Cbz in patients with metastatic or locally advanced solid tumors and varying degrees of hepatic impairment (NCT01140607) included a cohort with normal hepatic function to assess the effect of a single Cbz dose on the PK profile of a single dose of Mdz. This was an open-label, two-period, fixed-sequence study in patients aged between 45 and 60 years with advanced solid tumors and normal hepatic function. A single dose of Mdz (2 mg) was administered orally alone (Day –1) and at the end of a 1-hour infusion of Cbz (25 mg/m²) (Day 1), with a 24-hour interval between the two administrations of Mdz. Endpoints included AUC and AUC_lastof Mdz with and without Cbz administration, and safety evaluations. Results: Of the 13 patients enrolled and treated in the cohort, 11 patients were included in the PK analysis. Exposure (AUC and AUC_last) and other PK parameters after a single administration of Mdz alone and in combination with Cbz (Day 1) were similar. The AUC ratio for Mdz administered alone or with Cbz was 0.97 (90% CI: 0.76–1.23). The AUC_last ratio for Mdz administered alone or with Cbz was 1.04 (90% CI: 0.81–1.34). All 13 patients had ≥1 adverse event (AE), 11 (84.6%) experienced a Grade 3–4 AE, and 4 (30.8%) experienced a serious AE. The majority of Grade 3–4 AEs were haematological and no new or unexpected safety findings were observed. Conclusions: In this study, Cbz did not increase the plasma exposure of Mdz. This indicates that Cbz is not a CYP3A inhibitor in the clinical setting and can be administered in combination with drugs metabolized by CYP3A. Clinical trial information: NCT01140607.