This study examined whether the method of hair cortisol analysis is applicable to orang-utans (Pongo spp.) and can help to advance the objective monitoring of stress in non-human primates. Specifically, we examined whether fundamental prerequisites for hair cortisol analysis are given in orang-utans and, subsequently, whether segmental hair analysis may provide a retrospective calendar of long-term cortisol levels. For this, hair samples were examined from 71 zoo-living orang-utans (38 males, mean age=22.5years; 33 females, mean age=24years) for which detailed records of past living conditions were available. Hair samples were cut from defined body regions and were analyzed either in full length or in segments. Results showed that hair cortisol concentrations (HCC) were unrelated to age or sex of the individual animal. HCC were found to be higher in orang-utans, with perceived long-term stressful periods (mean HCC=43.6±26.5pg/mg, n=13) compared to animals without perceived stressful periods (19.3±5.5pg/mg, n=55, P<0.001). In non-stressed animals, segmental hair analyses revealed that HCC was stable along the hair shaft even when hair reached >40cm. The possibility of obtaining a retrospective calendar of stress-related cortisol changes through hair analysis was further supported by data of three case studies showing close correspondence between the segmental HCC results and keeper reports of stress exposure during the respective time periods. Finally, low within-animal variation in HCC from different body regions (CV%: 14.3) suggested that this method may also be applicable to naturally shed hair, e.g., as found in nests of wild orang-utans and other great apes. Therefore, using HCC may provide an ideal non-invasive tool for both captive management as well as conservation in orang-utans and potentially other great apes.