Intermittent hypoxic therapy (IHT) is a form of simulated altitude training which has been proposed to lead to a performance benefit in sport (Rodriguez et al., 1999: Medicine and Science in Sport and Exercise, 31(2), 246 – 248). Bailey et al. (2000: British Journal of Sports Medicine, 34, 210 – 212) have shown a reduction in ill health in elite athletes treated with IHT and suggested that IHT may be beneficial to immune function using glutamine levels as a marker. However, the use of glutamine levels as a marker of immune functions remains controversial (Hiscock and Pederson, 2002: Journal of Applied Physiology, 93, 313 – 322). The purpose of this preliminary study was to investigate whether regular exposure to IHT alters the haematology of highly trained athletes. The study was carried out using a 10 person altitude simulator (SportO2, Altitude Science, Auckland, New Zealand). Fourteen professional rugby club players (mean ± s: age 20 ± 2.3 years, height 1.78 ± 2.6m, body mass 94.8 ± 15.4kg) were asked to attend 15, once hour sessions daily in as short a period as possible. Eight subjects attended 12 – 15 sessions over a 25 day period. Six subjects were asked to discontinue the exposure when they missed 4 or more sessions in the first 8 days and acted as controls. During IHT, oxygen saturation levels in the blood were monitored using a Nellcor NPB-40 Handheld Pulse Oximeter throughout the exposure period. When the SpO2 dropped towards the minimum level for that day subjects were asked to breathe room air until SpO2 started to rise. Venous blood samples were taken 48 hours prior to the first exposure and 7 days following the last exposure. Samples were analysed for the determinations of absolute haemoglobin, haematocrit, total leucocytes, neutrophils, lymphocytes, monocytes, Natural Killer cells, B lymphocytes and T Lymphocytes. Due to low subject numbers it was decided to compare pre and post results using a Mann-Whitney test to determine if a systematic bias was observed. A significance level of P<0.02 (two tailed) was adopted. No differences were found for any of the variables measured for either the IHT group or the control group. In conclusion, 12 – 15 exposures to IHT for 60 min over a 25 day period did not result in a significant change in participant’s haematology. However due to the applied nature of the study, there were a number of confounding variables (eg. Poor subject adherence leading to lower subject numbers, lack of control over physical training and diet) which means that further studies are required before any judgement about how IHT affects haematological status can be confirmed. It remains debateable whether IHT confers any haematological adaptations which would result in a performance or health related benefit.