The cynomolgus monkey (Macaca fascicularis) has been increasingly used in biomedical research. Although living conditions affect behavioral and physiological characteristics in macaques, little data is available on how living conditions influence blood-based parameters in the cynomolgus monkey. We hypothesize that there are significant differences in serum biochemical and hematological parameters in single-caged versus socially housed cynomolgus monkeys, and that age and sex influence the effect of living conditions on these parameters. Sixty single-caged and 60 socially housed cynomolgus monkeys were segregated by age group (juvenile, adult) and sex. The effects of living condition, age, sex, and the interactions between these factors on commonly reported serum biochemical and hematological parameters were analyzed by a three-way analysis of variance (ANOVA). Then, the differences between single-caged and socially housed subjects were tested in each parameter by Student's t-test. Creatinine, glucose, triglyceride, alanine aminotransferase, red blood cell volume distribution width (SD, CV), median fluorescence reticulocyte percentage, white blood cell and basophil counts, and monocyte (count, %) were lower in single-caged subjects. Blood urea nitrogen and globulin were lower in single-caged juveniles and adults, respectively. Red blood cell count, hemoglobin, hematocrit, and neutrophil (count, %) were higher, and reticulocyte and lymphocyte (counts, %) were lower, in single-caged juveniles. Mean corpuscular hemoglobin concentration was higher in single-caged subjects (but more pronounced in adults). Total protein was higher in single-caged juvenile males and lower in single-caged adult females. Alkaline phosphatase was lower in single-caged juvenile females. Mean corpuscular hemoglobin was higher, and high fluorescence reticulocyte percentage was lower, in single-caged adult males. In conclusion, living conditions significantly affect several serum biochemical and hematological parameters in the cynomolgus monkey, and these effects vary by age and sex. As this macaque is commonly housed under different living conditions, these findings should aid researchers in avoiding inaccurate conclusions concerning this species. Am. J. Primatol. 76:1011–1024, 2014. © 2014 Wiley Periodicals, Inc.