Background: Exemestane, an aromatase inhibitor commonly used for breast cancer treatment, shares structural similarities with sex steroids analyzed in clinical laboratories. We aimed to investigate the influence of exemestane cross-reactivity in the measurement of sex steroids across various immunoassays. Methods: We conducted a multicenter study involving measurements of androstenedione, testosterone, estradiol, progesterone, and 17-hydroxyprogesterone in serum samples from women undergoing exemestane therapy (N = 15; 25 mg/day). Measurements were performed using liquid chromatography-mass spectrometry (LC-MS) and various commercially available chemiluminescence immunoassays, ELISA, and radioimmunoassay. In-vitro cross-reactivity was assessed by adding exemestane and 17-hydroexemestane to serum samples. Results: Patients undergoing exemestane therapy had markedly falsely elevated androstenedione results in all immunoassays evaluated (N = 4), which correlated with serum exemestane levels. In-vitro experiments confirmed this interference to be caused by cross-reactivity with exemestane. Additionally, one immunoassay yielded falsely elevated estradiol results in 20% of patients. However, in-vitro experiments did not confirm this to be caused by cross-reactivity with exemestane or 17-hydroexemestane. Conclusions Exemestane cross-reacts with androstenedione immunoassays, causing falsely elevated results in treated patients. This analytical interference may raise unnecessary concerns, leading to expensive diagnostic workups.