High serum levels of procalcitonin (PCT) are observed in patients with sepsis or severe infection. In a prospective study of 122 hospitalised adult medical patients with sepsis, serum PCT was determined on admission and for 9 days thereafter. Patients with no alteration in their immune system showed high PCT values up to day 5, decreasing to normal levels by day 9. Patients with sepsis and immunodeficiency had high values on days 0 to 2, similar to the first group, but showed significantly lower levels on the following 3 days. PCT concentrations fell to base line levels on days 6 to 9 of the sepsis episode in both groups. The observed difference was not significantly related to the kind of causative microorganism or a culture negative sepsis. Leukopenia seemed to go together with lower PCT values after day 2 of the episode, but this could not be proven statistically.