Remarkable crustal deformation has repeatedly been observed in the eastern Izu Peninsula, central Japan, synchronized to the occurrence of extensive earthquake swarms in the past 20 years. Among the instrumentation, a borehole tiltmeter installed at station KWN, which is located within a few kilometers of the swarm region, has faithfully recorded large signals of ground tilt associated with more than 10 swarm sequences in the past 10 years. In addition to coswarm signals it became clear that precursory tilt signals preceded major swarm activities. The amplitudes of the precursory tilt signals are of order of 0.1 μrad, and they precede the onset of major swarms by several hours to half a day. This signal is convincing because the same phenomena appeared at least 7 times and were also detected by other independent observations. We speculate that a dike intrusion at depth causes crustal deformation and only as it reaches depths shallower than ~10 km does seismic activity begin. The precursory tilt signal can be utilized for practical prediction of the start of swarm activity in the eastern Izu Peninsula. ¿ 2000 American Geophysical Union