Introduction: High altitude pulmonary oedema (HAPE) in severe cases is fatal. Altitude, ascent rate, degree of pre-acclimatisation and individual susceptibility determine overall risk of HAPE. Combining a fast ascent rate with high individual susceptibility leads to a HAPE incidence of 60% at 4559m (Bartsch et al. 2003). Elevated pulmonary artery systolic pressure (PASP) precedes the development of HAPE (Maggiorini, 2006). Normobaric hypoxia has been used as a method of identifying those individuals susceptible to HAPE using PASP as a marker of risk (Dehnert et al. 2005). However, acclimation or pre-acclimatisation techniques such as intermittent hypoxic exposure (IHE) and training (IHT) have not been evaluated as a method to reduce HAPE susceptibility. Methods PASP changes were assessed using IHE (n=10) and IHT (n=12) in comparison to a sea level control group (SLC, n=10) using 5 days of 5 hours.day-1 of normobaric hypoxia (4800m, day 1, 4, 5 and 4300m days 2 & 3). Echocardiography was used to measure PASP at four separate time points on days 1 and 5 (T1 = 0, T2 = 1.75hr, T3 = 4hrs, T4 = 5hrs). IHT and SLC walked at a gradient of 10 – 15%, carrying a 10kg load for 90 minutes from 2.5hrs – 4hrs, at relative intensities of 40 – 70% of altitude specific VO2max. The Ministry of Defence Research Ethics Committee approved the study. A three way repeated measures ANOVA (RMANOVA) was used to assess group differences and interactions, with effect size assessed using partial eta squared (η2). Independent and paired t-tests with Cohens d were conducted post-hoc to establish specific differences within and between groups. Results RMANOVA revealed a large significant interaction between day, time and group (P = 0,001, η2 = 0.196) indicating each group produced a different pattern of response. SLC had consistent PASP at all time points comparing day 1 and 5. Hypoxia caused a significant increase for both IHT and IHE on day 1 (mean ± SD: 22 ± 4 mmHg, P = 0.000, d = 5.63; 19 ± 7 mmHg, P = 0.000, d = 2.82). There was no significant effect of exercise on PASP for IHT. There were no significant changes in PASP comparing day 1 and 5 for IHE (P = 0.863, d = 0.06). In contrast, IHT showed a significantly reduced PASP at the end of day 5 compared to day 1 (mean ± SD: 5 ± 4 mmHg, P = 0.008, d = 1.33). Conclusion Acclimation using IHT has the potential to reduce HAPE susceptibility for individuals with previous history or military personal exposed to considerable terrestrial altitudes without sufficient time to acclimatise. Further research is required to fully establish its efficacy at terrestrial altitude and understand the mechanisms responsible for differences in PASP response to IHE and IHT.