We investigated precursory seismicity in and around the epicentral zone of the M-w = 7.4 1999 August 17 Izmit (Turkey) earthquake, by applying a statistical method-the RTL (Region-Time-Length) algorithm-to earthquake catalogues derived from that for the period 19811999 of Kandilli Observatory and Earthquake Research Institute (KOERI). The derived catalogues are complete for events MD 3 in most of western Turkey. After declustering aftershocks, we investigated the seismicity patterns preceding the Izmit event at local (Izmit tectonic zone) and national (Turkey) scales. The RTL parameter indicates that a period of seismic quiescence started at the end of 1995 and reached a minimum in December 1996. An activation phase lasting about three months followed. The main shock in Izmit and vicinity did not occur when the seismicity returned to its background level, but occurred with a delay of nearly 2.5 yr. We present a new parameter to quantify the spatial distribution of seismic quiescence. The results from both catalogues indicate that a significant quiescence anomaly appeared in 1996 around the epicenter of the Izmit earthquake. The primary characteristics of the seismicity patterns prior to the Izmit earthquake are similar to those obtained for large events in Russia and Japan. The variations of seismicity patterns revealed by the RTL algorithm may offer better understanding of the physical nature of seismo-tectonics and provide useful information for seismic hazard estimation. The varying characteristics of the Izmit and other events may reflect the difference between seismo-tectonics in Turkey and in other regions such as Russia and Japan.