Background: Biochemical monitoring of classic 21-hydroxylase-deficient congenital adrenal hyperplasia (CAH) relies on serum measurements of 17-hydroxyprogesterone (17OHP), androstenedione (A4) and testosterone (T). High biological variability, particularly of 17OHP, limits interpretation of results and makes therapeutic adjustments based on single measurements challenging (1). Aims: We aimed to investigate the biological variability of the 11-oxyandrogens and 21-deoxycortisol (21DF) and to compare steroid concentrations between poorly and adequately controlled patient samples as defined by serum A4 concentrations. Method: In this prospective study, repeated LC-MS/MS androgen profiles were performed in 21 women with classical CAH referred by Endocrinologists from the Keogh Institute, WA, between April 2022 and June 2025. The biological component of the intra-individual coefficient of variation (CVB) was calculated for each analyte. Serum concentrations of four 11-oxyandrogens and 21DF were compared between samples in adequate versus poor biochemical control as judged by A4 levels within or above age and sex appropriate reference ranges. Results: There were 74 observations among the 21 participants. The CVB was lowest for 11OHT (33.7%) and 11KA4 (41.8%) and highest for 21DF (157.5%). Of the conventional biomarkers, CVB was lowest for T (47.0%) and A4 (60.5%) and highest for 17OHP (107.9%). Median concentrations of all steroids were lower in adequately versus poorly controlled samples (all p-values <0.0001). When A4 was within range, all four 11-oxyandrogens and T were within published normative ranges (2). However, 17OHP and 21DF remained elevated in adequately controlled samples. Conclusions: These results support recommendations to aim for A4 and T within the upper-normal range while allowing 17OHP and 21DF to remain elevated to avoid glucocorticoid over-replacement. The 11-oxyandrogens may be superior to conventional biomarkers for therapeutic monitoring given their lower biological variability and adrenal-specificity. Normalising 11KT, a potent adrenal-specific androgen may be a reasonable therapeutic goal. Larger studies including children and men are required.