A piezoelectric immunosensor we developed for microalbumin with an anti-human albumin-immobilized crystal was sufficiently sensitive to monitor the levels of microalbumin in urine. The sensor does not require any radioisotope- or enzyme-labeled antibody, a second antibody, which is essential for ordinary immunoassay such as an immunoradiometric assay (IRMA) or an enzyme-linked immunosorbent assay (ELISA). We also developed a latex piezoelectric immunoassay (LPEIA), which requires no immobilization of an antigen or antibody on the surface of piezoelectric quartz crystal. LPEIA was used to detect C-reactive protein (CRP), antistreptolysin O (ASO) and rheumatoid factor (RF), and was sufficiently sensitive for clinical applications. The principles, merits and limitations of these sensors are discussed in this review.