A prospective study was performed over a 4.5-year period to determine the ability of a sputum Gram stain to predict the cause of community-acquired bacterial pneumonia. A blood culture isolate, rather than a sputum culture, served as the reference standard to provide precise identification of the etiologic agent. The study population comprised 59 bacteremic adults who expectorated a valid sputum sample. Data are presented that indicate that a physician, aided by the morphology of the stained sputum, could theoretically select appropriate monotherapy approximately 94% of the time when selective, defined criteria for the microbiology of valid sputum are met. Three of the five patients with pneumonia caused by Haemophilus influenzae, however, had sputum stains that suggested alternative pathogens. This study reaffirms that the Gram-stained sputum is a reliable, but not infallible, guide to direct initial antibiotic therapy in adults with community-acquired bacterial pneumonia.