Pneumonia complicating abdominal sepsis. An independent risk factor for mortality. Nosocomial pneumonia (NP) is associated with a significant mortality, 66% in a previous retrospective study of NP complicating intra-abdominal sepsis (IAS). We prospectively compared the outcome of NP complicating IAS with that of recurrent IAS (R-IAS) in the absence of NP. Data were collected prospectively on 300 patients with IAS; 34 patients who presented with pneumonia were excluded from the analysis (44% mortality). One hundred seventy-one patients with no NP and no R-IAS (group 1) had a hospital mortality of 20% (34 patients); 36 without NP in whom R-IAS developed (group 2) had a 17% mortality (six patients); and 47 with NP but no R-IAS (group 3) had a 53% mortality (25 patients). Finally, 12 patients who had both NP and R-IAS suffered a 75% mortality (nine patients). We examined the relationships among the following putative risk factors and mortality: APACHE (acute physiology and chronic health evaluation) II score (at initial presentation with IAS), the need for mechanical ventilatory assistance following initial treatment for peritonitis, steroid requirement, generalized peritonitis vs abscess, and the need for surgical as opposed to percutaneous treatment. Using mortality as the dependent variable, group 2 vs 3 as the explanatory variable, and the risk factors as confounders, logistic regression analysis indicated that the group difference was significant after controlling for confounders. We conclude that NP complicating IAS is an independent risk factor associated with a significant mortality compared with R-IAS. These data challenge the notion that death in IAS is usually due to recurrent or persistent intra-abdominal infection. 