This paper describes the development of a highly sensitive competitive immunoassay with the piezoelectric sensor. The immobilized derivative of cocaine was benzoylecgonine-1,8-diamino-3,4-dioxaoctane (BZE-DADOO). For the immobilization of BZE-DADOO, the conjugate BZE-DADOO with 11-mercaptomonoundecanoic acid (MUA) was synthesized via 2-(5-norbornen-2,3-dicarboximide)-1,1,3,3-tetramethyluronium-tetrafluoroborate (TNTU), followed by the creation of the conjugate monolayer on the piezosensor electrodes. For the optimization of the competitive assay we used electrodes with rough or smooth gold areas and for the interaction with immobilized antigen different anti-cocaine sheep polyclonal (pAb, either whole IgG or Fab fragment) and mouse monoclonal (mAb, whole IgG) antibodies. The assay of cocaine developed achieved a detection limit (LOD) of 100 pmol/l (34 ng/l) using the sheep antibody (IgG) and piezoelectric sensors with a smooth gold surface. The total time of one analysis was 15 min and the measuring area of the sensor could be used more than 40 times without losing its sensitivity.