In up to a third of patients admitted to hospital with community acquired pneumonia the microbial a etiology is never determined. Possibly some of these infections are due to currently unknown pathogens. To understand the a etiology of these undetermined cases better, we have compared the presenting clinical features of these patients with those of patients with an identified microbial a etiology. On the assumption that some of the unidentified cases might be due to recognised microbial pathogens, a discriminant function analysis was used to show the a etiological category into which the undetermined cases would best fit.