Systemic inflammation has been linked to suppressed CYP3A(4) activity. We determined 4β-hydroxycholesterol (4βOHC), an endogenous CYP3A4 metabolite, in patients with rheumatoid arthritis (RA) before and after treatment with biological disease-modifying antirheumatic drugs (bDMARDs). The 4βOHC was compared in 41 patients before and 2-5 months after initiating TNFα inhibitors (n = 31), IL-6 inhibitors (n = 5), or B-cell inhibitors (n = 5). Correlations between 4βOHC and inflammatory markers (C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR)) were also tested before and after bDMARDs. 4βOHC did not differ following bDMARD treatment (P = 0.6), nor in patients who started with IL-6 inhibitors (median 51.6 vs. 50.6 nmol/L). The 4βOHC and CRP/ESR did not correlate before treatment (P textgreater 0.5), but correlated significantly after bDMARDs (CRP = Spearman r -0.40; P textless 0.01; ESR = r -0.34; P = 0.028) suggesting that mainly non-CYP3A4-suppressive cytokines were reduced during treatment. Thus, this study does not support a generally regained CYP3A4 phenotype in patients with RA following initiation of bDMARDs.