There is conflicting evidence whether hypoxia improves running economy (RE), maximal O 2 uptake (V˙O2 max), haemoglobin mass (Hb mass ) and performance, and what total accumulated dose is necessary for effective adaptation. The aim of this study was to determine the effect of an extended hypoxic exposure on these physiological and performance measures. Nine elite middle distance runners were randomly assigned to a live high–train low simulated altitude group (ALT) and spent 46±8 nights (mean±S.D.) at 2860±41m. A matched control group (CON, n=9) lived and trained near sea level (∼600m). ALT decreased submaximal V˙O2 (Lmin −1 ) (−3.2%, 90% confidence intervals, −1.0% to −5.2%, p=0.02), increased Hb mass (4.9%, 2.3–7.6%, p=0.01), decreased submaximal heart rate (−3.1%, −1.8% to −4.4%, p=0.00) and had a trivial increase in V˙O2 max (1.5%, −1.6 to 4.8; p=0.41) compared with CON. There was a trivial correlation between change in Hb mass and change in V˙O2 max (r=0.04, p=0.93). Hypoxic exposure of ∼400h was sufficient to improve Hb mass , a response not observed with shorter exposures. Although total O 2 carrying capacity was improved, the mechanism(s) to explain the lack of proportionate increase in V˙O2 max were not identified.