The endemic Javan hawk-eagle Spizaetus bartelsi is considered threatened with extinction because of its small population size and fragmentation of its habitat on the densely populated island of Java, Indonesia. Like many other tropical forest raptors little is known about many of its population parameters. Research was carried out from 1980 to 2000 in order to assess the status of this species. Its presence was confirmed throughout the island in both wet and dry climatic zones. Home range sizes were calculated to range between 12-36 km2, and comparison with published estimates suggests that these may differ significantly between areas. Encounter rates are in the order of 0.1-0.9 birds per survey day, and were significantly higher in areas with a short dry season compared to areas with a long dry season. Based on field-observations, museum skins and captive birds, the adult: non adult ratio is 1: 1.3. An assessment of habitat quality for all large areas where Javan hawk-eagles have been recorded, and a conservative working density differentiated to habitat quality, lead us to estimate that there are 137-188 remaining pairs, which account for a total world population of just short of a thousand birds. We make a number of suggestions for further research aimed at obtaining more insight on dispersal, recruitment and age-related habitat preferences, and for improved conservation, including more strict law enforcement and gazettment of new reserves.