We compared the prevalence of Gram-negative bacilli in the pharyngeal flora of two groups of patients with a known predilection for Gram-negative bacillary pneumonia (chronic alcoholics and diabetics), two other groups of aspiration-prone persons with no known predilection for Gram-negative bacillary pneumonia (epileptics and narcotic addicts), and normal control subjects. Quantitative cultures of saline gargles showed pharyngeal Gram-negative bacilli to be significantly (P less than .05) more prevalent among alcoholics (35%) and diabetics (36%) but not epileptics (17%) or addicts (20%) than controls (18%). Counts of greater than or equal to 100 Gram-negative bacilli per milliliter were also significantly more common in alcoholics (14%) and diabetics (24%) than controls (5%, P less than .05). Enterobacter sp, Klebsiella pneumoniae, and Escherichia coli were the most common Gram-negative bacilli isolated. Increased colonization by Gram-negative bacilli might be a factor contributing to the propensity of alcoholics and diabetics for Gram-negative pneumonia.