The remarkable large-mammal fauna of the Indonesian island of Sumatra is one of the most endangered on Earth and is threatened by rampant deforestation. We used remote sensing and biological surveys to study the effects of deforestation on populations of endangered large mammals in a Sumatran landscape. We measured forest loss and created a predictive model of deforestation for Bukit Barisan Selatan National Park and an unprotected buffer area based on satellite images between 1985 and 1999. We used automatic cameras to determine the distribution and relative abundance of tigers ( Panthera tigris sumatrae ), elephants ( Elephas maximus ), rhinoceros ( Dicerorhinus sumatrensis ), and tapirs ( Tapir indicus ). Between 1985 and 1999, forest loss within the park averaged 2% per year. A total of 661 km (2) of forest disappeared inside the park, and 318 km (2) were lost in a 10-km buffer, eliminating forest outside the park. Lowland forest disappeared faster than hill/montane forest ( by a factor of 6 ) and forests on gentle slopes disappeared faster than forests on steep slopes ( by a factor of 16 ). Most forest conversion resulted from agricultural development, leading to predictions that by 2010 70% of the park will be in agriculture and that by 2036 lowland forest habitat will be eliminated. Camera-trap data indicated avoidance of forest boundaries by tigers, rhinoceroses ( up to 2 km ), and elephants ( up to 3 km ). Classification of forest into core and peripheral forest based on mammal distribution suggests that, by 2010, core forest area for tigers and rhinoceros will be fragmented and reduced to 20% of remaining forest. Core forest area for elephants will be reduced to 0.5% of remaining forest. Halting forest loss has proven one of the most difficult and complex problems faced by Indonesia's conservation agencies today and will require a mix of enforcement, wise land-use strategies, increased education, capacity to manage, and new financing mechanisms.