['Patients resuscitated from cardiac arrest have a high early mortality rate. Prognostic evaluation based on clinical observations is uncertain and would benefit from the use of biochemical markers of hypoxic brain damage. The astroglial protein S-100 is an established biochemical marker of central nervous system injury. The purpose of the present study was to validate the use of serum determinations of S-100 with regard to outcome after cardiac arrest.', 'Levels of serum S-100 were measured with a radioimmunoassay in 41 patients the first 3 days after out-of-hospital cardiac arrest. The main outcome variable was fatal outcome within 14 days.', 'S-100 levels were increased after cardiac arrest compared with controls with the highest levels observed the first day. S-100 levels day 1 and 2 correlated to the degree of coma as well as to the time of anoxia. Seventeen patients died within 14 days after the cardiac arrest. The deceased patients had increased S-100 levels on days 1 through 3 compared with survivors. All patients (100%) with an S-100 level of > or =0.2 on day 2 after the cardiac arrest died within 14 days, and 89% of the patients with levels below this limit value survived (positive and negative predictive values). The corresponding predictive values on day 1 were 71% and 85%, respectively.'] Are increased serum levels of the S-100 protein associated with hypoxic brain damage after cardiac arrest?