Pseudomonas species cause a specific necrotizing pneumonia that may be recognized by histologic appearance. In 1963, this lesion was present in 41 of 522 autopsied patients (7.9%). When the hospital course of this group was compared to the course of a similar group of autopsied control patients, it was found that reservoir nebuliza-tion, penicillin, "anti-gram positive" agents, and broad spectrum antimicrobials were utilized significantly more frequently in the pneumonia patients than in the control patients. Steroids were utilized more frequently in the pneumonia patients, but the level of significance was borderline. In addition, shock and anemia had been significantly more frequent in the pneumonia patients than in the control patients. A seriously ill patient may develop gram-negative bacillary pneumonia because of the alteration of host flora by antimicrobial therapy followed by the inoculation of large numbers of gram-negative bacilli by contaminated aerosol. The presence of shock or anemia may further increase the likelihood of developing necrosis.