We report on the change in brain oxygen tension (PbtO2) over the first 24 h of monitoring in a series of 25 patients with severe traumatic brain injury (TBI) and relate this to outcome. The trend in PbtO2 for the whole group was to increase with time (mean PbtO2 17.4 [1.75] vs 24.7 [1.60] mmHg, first- vs last-hour data, respectively; p = 0.002). However, a significant increase in PbtO2 occurred in only 17 patients (68 %), all surviving to intensive care unit discharge (p = 0.006). Similarly, a consistent increase in PbtO2 with time occurred in only 13 patients, the correlation coefficient for PbtO2 versus time being ≥0.5 for all survivors. There were eight survivors and four non-survivors, with low correlation coefficients (<0.5). Significantly more patients with a correlation coefficient ≥0.5 for PbtO2 versus time survived in intensive care (p = 0.039). The cumulative length of time that PbtO2 was <20 mmHg was not significantly different among these three groups. In conclusion, although for the cohort as a whole PbtO2 increased over the first 24 h, the individual trends of PbtO2 were related to outcome. There was a significant association between improving PbtO2 and survival, despite these patients having cumulative durations of hypoxia similar to those of non-survivors.