A new methodology, sandwich-type monitoring assay system using an estrogen response element (ERE)-immobilized piezoelectric biosensor in combination with flow injection technique, has been developed for analysis of estrogen (E). The principle of the assay is that the estrogen receptor (ER) captures estrogen and then the complex is bound with ERE immobilized on the sensor. It was confirmed that initial binding of the ERE with ER is significantly accelerated by the formation of an E–ER complex. This kinetic difference was monitored by real-time measurement of the resonance frequency of the ERE piezoelectric biosensor and applied to detect 17β-estradiol. The calibration graph was linear in the range 10–100 nmol l^–1 with a detection limit of 7.8 nmol l^–1, an RSD of 5.9% for 50 nmol l^–1 (6 replicates), and one run time of 4 min. Gradient flow injection on-line connection to an array of ERE piezoelectric biosensors is in progress for the simultaneous determination of endogenous and synthetic estrogens in drinking water and urine. In comparison with present chromatographic methods coupled with MS, the proposed technique is simple, flexible and cheap and has great potential in applications such as field and on-site monitoring and screen testing of endocrine disruptors.