The reason for the existence of the tourist resort is the Bukit Lawang Bohorok Orang-Utan Rehabilitation Centre (whttp://www.orangutans-sos.org), founded in 1973 by two Swiss women, Monica Borner and Regina Frey, with the aim of returning captive and orphaned orang-utans into the wild after re-teaching them the art of tree climbing and nest building. The rehabilitation programme was suspended a while ago and though the centre is still open, it is more a tourist attraction than anything else.
Visitors are allowed to watch the twice-daily, hour-long feeding sessions that take place on the hill behind the centre. All visitors must have a permit from the PHPA office. All being well, you should see at least one orang-utan during the session, and to witness their gymnastics is to enjoy one of the most memorable experiences in Indonesia.