Inflammation may play a central role in the pathogenesis of HIV-related Pneumocystis carinii pneumonia (PCP). Serum levels of the amino-terminal propeptide of Type III procollagen (PIIINP) reflect inflammatory activity in granulation tissue and in chronic rheumatic and liver disorders. To investigate changes in PIIINP serum levels during an episode of HIV-related PCP, consecutive serum samples were taken from 48 HIV-infected patients with PCP in a randomized, placebo-controlled study of the effect of adjunctive methyl-prednisolone therapy (26 in corticosteroid [CS] group and 22 in control group). All patients were treated with co-trimoxazole. In the control group, PIIINP serum levels at day of initiation of therapy (Day 0) were significantly higher in patients requiring mechanical ventilation and/or dying during the course of the pneumonia, and serum levels of PIIINP higher than 5 ng/ml were associated with a higher mortality than levels below 5 ng/ml. The level of PIIINP increased from Day 0 to Day 5. There was a significant correlation between changes in PIIINP levels and changes in the alveolar-arterial oxygen gradient from Day 0 to Day 5. In the CS group, the PIIINP levels decreased while steroid was administered. At Days 21 to 28 there were no difference in the levels of PIIINP between the two groups. PIIINP serum levels may predict the clinical outcome of PCP. The antimicrobial therapy may exacerbate the inflammatory reaction in HIV-related PCP, leading to respiratory failure. CS prevents this increased inflammatory activity.