The activation of macrophages is accompanied by release of 2-amino-4-oxo-6(D-erythro-l′,2′,3′-trihy-droxypropyl)-dihydropteridine (D-erythro-neopterin). The neopterin levels of 21 patients were measured with radioimmunoassay. The patients were classified according to the clinical course and outcome. We found highly significant differences between survivors and nonsurvivors for each of the evaluated days of the observation period. In addition to a sustained increase, patients with fatal outcome always showed a higher percentage of neopterin levels (88.2 ± 28 [sd]%) exceeding the upper confidence limit (27.4 nmol/L) than survivors (31.8 ± 29.9%). We conclude that the assessment of D-erythro-neopterin might be an easily available aid for an early evaluation of the immunologic status of a patient at risk for septic complications.