<p>Biomarkers in body fluids provide essential chemical information for examining health conditions; however, unlike conventional instrumental approaches, easy-to-use analytical methods have not yet been fully established. This review introduces extended-gate-type organic field-effect transistors (OFETs) as biosensor platforms for real-sample analysis. OFETs are electronic devices that show switching profiles when gate voltages are applied. Therefore, the gate electrode of OFET functions as a sensing unit combined with appropriate molecular recognition materials. Owing to their signal amplification properties, OFETs enable sensitive biosensing. The extended-gate surfaces are easily functionalized with enzymatic layers using chemical modification, and these surfaces provide a high discrimination ability for specific biomarkers from their analogs. This review presents the designs of the extended-gate structures (i.e., integrated and separated styles) and their enzymatic layers and includes their actual sensing performance.</p> Graphical abstract <p></p>