BACKGROUND The optimal rewarming technique for patients in deep accidental hypothermia with core temperatures below 28 degrees C is not established. Several authors believe that extracorporeal rewarming is essential, especially for patients with hemodynamic instability. Others believe that invasive rewarming ought to be reserved for patients in cardiac arrest. We describe our experience with a strictly conservative technique without the use of invasive rewarming devices in patients with severe accidental hypothermia and a sustained perfusion rhythm. METHODS A cohort study extending from 1991 to 2000, including all patients received at the emergency department of the University Hospital of Vienna with severe hypothermia, a core temperature of maximum 28 degrees C and no preclinical cardiac arrest. RESULTS 36 patients with deep hypothermia were included in the study. Their core temperatures ranged from 20.2 degrees C to 28 degrees C; the median temperature was 25.75 degrees C (25th and 75th percentile, 24.2/27.3). Fourteen patients were intoxicated and their multimorbidity was high. All of 19 patients with stable hemodynamics and 14 of 17 patients with unstable hemodynamics were successfully rewarmed to normothermia with warmed infusions, inhalation rewarming and forced air rewarming. The rewarming process took 9.5 hours (8/10.5) and required a volume load of 4820 ml (2735/5770). The rewarming rate was 1.09 degrees C per hour (0.94/1.25). Although 92% of the patients were successfully rewarmed to normothermia, in-hospital mortality was 42%, but was largely related to comorbidity. CONCLUSIONS A conservative approach is highly successful in achieving normothermia in patients with deep hypothermia with or without stable hemodynamics. In-hospital mortality of severe accidental hypothermia in urban conditions is high; comorbidity might play a major role. The influence of the rewarming strategy on late in-hospital mortality remains unclear.