There is a paucity of basic biological information for the Sumatran rhinoceros (Dicerorhinus sumatrensis). This information is fundamental to husbandry and management practices for captive animals and for support of in situ conservation efforts. Serial blood samples were collected over an 8-yr period to evaluate patterns in hematology and serum biochemistry values among five Sumatran rhinoceroses housed at the Sumatran Rhino Sanctuary in Way Kambas National Park, Lampung, Indonesia. Understanding the basis for variance in analytes both within and between subjects can allow use of more sensitive subject-based reference values, and is particularly suitable for small populations of endangered animals. Both intra- and intersubject variability was computed for each analyte and the associated index of individuality was determined. Previously published cutoff points for index of individuality indicate where population-based reference intervals can be used with confidence (index >1.4) or with caution (0.6 < index < 1.4). Interrhino variability was small for the majority of analytes, with 12 of 19 analytes having an index of individuality greater than 1.4 and none having an index of individuality less than 0.6. With the high within-individual variability of most anayltes in the Sumatran rhinoceroses at the sanctuary, subject-based reference intervals offer little advantage over standard population-based reference intervals for monitoring the health of these endangered animals. Differences were noted (but not tested for statistical significance) in serum urea, aspartate aminotransferase (AST), lactate dehydrogenase (LDH), and cholesterol between young and old rhinoceroses, and in hematocrit, AST, alanine aminotransferase (ALT), LDH, and glucose between male and female rhinoceroses. Husbandry practices, animal management, nutrition and habitat factors may also impact hematology and biochemistry results, and these relationships deserve more careful investigation. This study represents the most comprehensive hematology and serum biochemistry comparison of Sumatran rhinoceroses held in natural rainforest conditions outside a traditional zoological setting.