A relationship between physiological parameters of severe sepsis and immunological function has not been established. In ten severely ill patients with sepsis physiological risk was assessed by the Acute Physiology and Chronic Health Evaluation (APACHE) III score, while one component of immunological function was evaluated using peripheral blood mononuclear cell (PBMC) cytokine production after stimulation with lipopolysaccharide (LPS) in vitro. Five of the ten patients died. Mean (s.e.m.) APACHE III scores at admission were not significantly different between survivors and non-survivors (82(13) versus 95(13)) but after 72 h they were lower in survivors (51(13) versus 111(15), P < 0.05). Downregulation of cytokine production by PBMC on LPS stimulation was a transient event in survivors. Survivors had a three-fold increase in tumour necrosis factor alpha bioactivity within 72 h, but there was no increase in non-survivors. A similar pattern was demonstrated for interleukin (IL) 1 beta (P < 0.05 between survivors and non-survivors) and IL-6 (P = 0.06) immunoactivity. Physiological as well as immunological parameters in critically ill patients with sepsis independently predicted hospital survival (r2 = 0.2). These data demonstrate a relationship between the pattern of cytokine production in vitro and survival.