Thermal damage to host bone is a possible source of compromise of fixation in patients undergoing cemented total hip replacement (THR). Data on the subject to date are derived from mathematical modelling powered by animal studies. The aim of this study was to assess the effect of cement thickness on osteocyte viability in a population of patients undergoing cemented THR. An in vivo model was designed and validated by means of a finite element analysis. During standard hip joint replacement in 14 patients, the femoral necks were exposed before final resection to the heat of a curing cement mantle equivalent to 2.5 (Group 1) or 5 mm (Group 2) in vivo in the cemented acetabulum. Matched controls were collected for each patient. Osteocyte counts and viability were assessed by means of haematoxylin and eosin (H&E) stain and lactate dehydrogenase (LDH) assay. Ex vivo experiments were performed to determine the extent of thermal insult. H&E staining proved unreliable for assessing thermal insult in the short term. The LDH assay was reliable and demonstrated a significant reduction in osteocyte viability to a depth of 2.19 mm in group 1 and 9.19 mm in group 2. There was a significant difference between the groups at all depths. The ex vivo experiments revealed thermoclines indicating that host bone in the population undergoing cemented THR is more sensitive to the thermal insult delivered by curing polymethylmethacrylate cement than previously believed. This thermal insult may weaken the fixation between bone and cement and contribute towards aseptic loosening, the commonest cause of failure of THRs.