We estimated Capercaillie Tetrao urogallus population dynamics in Augustów Forest (NE Poland) based on spring counts of males on leks. During the 15-year study, the number of active leks decreased from 12 to 6 only, despite establishing 4 new leks. In 1996, the mean Nearest Neighbor Distance between leks was 4.06 km and varied from 0.95 to 9.25 km, and in 2010, the NND increased to 7.51 km (range 2.60–18.30). In 1996–1998, the number of males (cocks) per lek varied from 1 to 16. In 2010, the number of displaying cocks declined to 1–5 per lek. The mean number of cocks per lek declined from 4.9 in 1996 to 2.7 in 2010. Our censuses recorded a decline in the total numbers of displaying males from 59 in 1996 to 16 in 2010. During the study, the mean rate of extinction was 3.14 males per year. Historical data about population dynamic and causes of bird mortality is also discussed.