BACKGROUND: Hypothermia has been used in the treatment of head injury for many years. Encouraging results from small trials and laboratory studies led to renewed interest in the area and some larger trials. OBJECTIVES: To estimate the effect of mild hypothermia for traumatic head injury on mortality and long-term functional outcome complications. SEARCH STRATEGY: We searched the Injuries Group Specialised Register, Current Controlled Trials MetaRegister of trials, Zetoc, ISI Web of Science: Science Citation Index Expanded (SCI-EXPANDED) and Conference Proceedings Citation Index-Science (CPCI-S), CENTRAL (The Cochrane Library), MEDLINE and EMBASE. We handsearched conference proceedings and checked reference lists of all relevant articles. The search was last updated in January 2009. SELECTION CRITERIA: Randomised controlled trials of hypothermia to a maximum of 35 degrees C for at least 12 consecutive hours versus control in patients with any closed traumatic head injury requiring hospitalisation. Two authors independently assessed all trials. DATA COLLECTION AND ANALYSIS: Data on death, Glasgow Outcome Scale and pneumonia were sought and extracted, either from published material or by contacting the investigators. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated for each trial on an intention-to-treat basis. MAIN RESULTS: We found 23 trials with a total of 1614 randomised patients. Twenty-one trials involving 1587 patients reported deaths. There were fewer deaths in patients treated with hypothermia than in the control group (OR 0.84, 95% CI 0.67 to 1.05). Nine trials with good allocation concealment showed no decrease in the likelihood of death compared with the control group, and this result was not statistically significant (OR 1.08, 95% CI 0.79 to 1.47). Twenty-one trials involving 1587 patients reported data on unfavourable outcomes (death, vegetative state or severe disability). Patients treated with hypothermia were less likely to have an unfavourable outcome than those in the control group (OR 0.76, 95% CI 0.61 to 0.93). Nine trials with good allocation concealment showed patients treated with hypothermia were less likely to have an unfavourable outcome than those in the control group, but the reduction was small and non-significant (OR 0.91, 95% CI 0.69 to 1.20). Hypothermia treatment was associated with a slight increase in the odds of pneumonia (OR 1.31, 95% CI 0.93 to 1.86) but there was a reduction in pneumonia for trials with good allocation concealment (4 trials analysed separately, 294 patients, OR 0.79, 95% CI 0.49 to 1.27) although in both cases the results are not statistically significant. AUTHORS' CONCLUSIONS: There is no evidence that hypothermia is beneficial in the treatment of head injury. Hypothermia may be effective in reducing death and unfavourable outcomes for traumatic head injured patients, but significant benefit was only found in low quality trials. Low quality trials have a tendency to overestimate the treatment effect. The high quality trials found no decrease in the likelihood of death with hypothermia, but this finding was not statistically significant and could be due to the play of chance. Hypothermia should not be used except in the context of a high quality randomised controlled trial with good allocation concealment.