OBJECTIVE To determine if the biologically active or bioavailable inhibin B (bio-inhB) correlated with the oocyte yield in controlled ovarian stimulation (COS). METHODS Cross-sectional study. METHODS Academic center. METHODS Women undergoing oocyte cryopreservation. METHODS None. METHODS Serum of women were sampled to measure bio-inhB at three points: baseline ("start"); middle ("mid"); and end of COS. A validated, highly specific enzyme-linked immunosorbent assay (Ansh Labs, Webster, TX) measured bio-inhB. The Spearman tests analyzed correlations between bio-inhB and other ovarian reserve markers, including age, follicle-stimulating hormone (FSH), antral follicle count (AFC), and antimüllerian hormone (AMH), and correlations between these markers and oocyte yield. RESULTS A total of 144 women were included. Bioavailable inhibin B at the mid and end of COS, plus its delta, were strongly correlated with other ovarian reserve markers. As the bio-inhB concentration increased, the AFC and AMH levels also increased, whereas the FSH concentration and age decreased. Bioavailable inhibin B values, except at the start of COS, were more strongly correlated with oocyte yield than the FSH concentration (r = 0.72-0.82 vs. r = -0.44) and correlated similarly to the AFC and AMH concentration (r = 0.79 and 0.81, respectively). These correlations strengthened in those with diminished ovarian reserve, specifically age ≥35 years or AMH concentration <2 ng/mL (r = 0.71-0.86 vs. r = 0.49-0.67). CONCLUSIONS Predicting COS outcome is imperfect. When using a highly specific enzyme-linked immunosorbent assay, bio-inhB correlated with the oocyte yield similar to or more strongly than traditionally used ovarian reserve markers. These correlations strengthened in cases of diminished ovarian reserve. Bioavailable inhibin B provides physicians with an additional clinical tool for estimating COS outcome.